usermods: Add SSD1306 display with u8g2

This commit adds example of usermod file for displaying basic
infromation SSD1306 I2C OLED displya utilizing u8g2 library.

Related to:
https://github.com/Aircoookie/WLED/issues/389

usermods/ssd1306_i2c_oled_u8g2/README.md

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+# SSD1306 128x32 OLED via I2C with u8g2
+This usermod allows to connect 128x32 Oled display to WLED controlled and show 
+the next information:
+- Current SSID
+- IP address if obtained
+  * in AP mode and turned off lightning AP password is shown
+- Current effect
+- Current palette
+- On/Off icon (sun/moon)
+
+## Hardware
+![Hardware connection](assets/hw_connection.png)
+
+## Requirements
+Functionality checked with:
+- commit 095429a7df4f9e2b34dd464f7bbfd068df6558eb
+- Wemos d1 mini
+- PlatformIO
+- Generic SSD1306 128x32 I2C OLED display from aliexpress
+
+### Platformio
+Add `U8g2@~2.27.2` dependency to `lib_deps_external` under `[common]` section in `platformio.ini`:
+```ini
+# platformio.ini
+...
+[common]
+...
+lib_deps_external =
+  ...
+  U8g2@~2.27.2
+...
+```
+
+### Arduino IDE
+Install library `U8g2 by oliver`  in `Tools | Include Library | Manage libraries` menu.

usermods/ssd1306_i2c_oled_u8g2/wled06_usermod.ino

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+#include <U8x8lib.h> // from https://github.com/olikraus/u8g2/
+
+// If display does not work or looks corrupted check the
+// constructor reference:
+// https://github.com/olikraus/u8g2/wiki/u8x8setupcpp
+// or check the gallery:
+// https://github.com/olikraus/u8g2/wiki/gallery
+U8X8_SSD1306_128X32_UNIVISION_HW_I2C u8x8(U8X8_PIN_NONE, 5,
+                                          4); // Pins are Reset, SCL, SDA
+
+// gets called once at boot. Do all initialization that doesn't depend on
+// network here
+void userSetup() {
+  u8x8.begin();
+  u8x8.setPowerSave(0);
+  u8x8.setFont(u8x8_font_chroma48medium8_r);
+  u8x8.drawString(0, 0, "Loading...");
+}
+
+// gets called every time WiFi is (re-)connected. Initialize own network
+// interfaces here
+void userConnected() {}
+
+// needRedraw marks if redraw is required to prevent often redrawing.
+bool needRedraw = true;
+
+// Next variables hold the previous known values to determine if redraw is
+// required.
+String knownSsid = "";
+IPAddress knownIp;
+uint8_t knownBrightness = 0;
+uint8_t knownMode = 0;
+uint8_t knownPalette = 0;
+
+long lastUpdate = 0;
+// How often we are redrawing screen
+#define USER_LOOP_REFRESH_RATE_MS 5000
+
+void userLoop() {
+
+  // Check if we time interval for redrawing passes.
+  if (millis() - lastUpdate < USER_LOOP_REFRESH_RATE_MS) {
+    return;
+  }
+  lastUpdate = millis();
+
+  // Check if values which are shown on display changed from the last tiem.
+  if ((apActive == true ? String(apSSID) : WiFi.SSID()) != knownSsid) {
+    needRedraw = true;
+  } else if (knownIp != (apActive ? IPAddress(4, 3, 2, 1) : WiFi.localIP())) {
+    needRedraw = true;
+  } else if (knownBrightness != bri) {
+    needRedraw = true;
+  } else if (knownMode != strip.getMode()) {
+    needRedraw = true;
+  } else if (knownPalette != strip.getSegment(0).palette) {
+    needRedraw = true;
+  }
+
+  if (!needRedraw) {
+    return;
+  }
+  needRedraw = false;
+
+  // Update last known values.
+  knownSsid = apActive ? WiFi.softAPSSID() : WiFi.SSID();
+  knownIp = apActive ? IPAddress(4, 3, 2, 1) : WiFi.localIP();
+  knownBrightness = bri;
+  knownMode = strip.getMode();
+  knownPalette = strip.getSegment(0).palette;
+
+  u8x8.clear();
+  u8x8.setFont(u8x8_font_chroma48medium8_r);
+
+  // First row with Wifi name
+  u8x8.setCursor(1, 0);
+  u8x8.print(ssid.substring(0, u8x8.getCols() > 1 ? u8x8.getCols() - 2 : 0));
+  // Print `~` char to indicate that SSID is longer, than owr dicplay
+  if (ssid.length() > u8x8.getCols())
+    u8x8.print("~");
+
+  // Second row with IP or Psssword
+  u8x8.setCursor(1, 1);
+  // Print password in AP mode and if led is OFF.
+  if (apActive && bri == 0)
+    u8x8.print(apPass);
+  else
+    u8x8.print(ip);
+
+  // Third row with mode name
+  u8x8.setCursor(2, 2);
+  uint8_t qComma = 0;
+  bool insideQuotes = false;
+  uint8_t printedChars = 0;
+  char singleJsonSymbol;
+  // Find the mode name in JSON
+  for (size_t i = 0; i < strlen_P(JSON_mode_names); i++) {
+    singleJsonSymbol = pgm_read_byte_near(JSON_mode_names + i);
+    switch (singleJsonSymbol) {
+    case '"':
+      insideQuotes = !insideQuotes;
+      break;
+    case '[':
+    case ']':
+      break;
+    case ',':
+      qComma++;
+    default:
+      if (!insideQuotes || (qComma != knownMode))
+        break;
+      u8x8.print(singleJsonSymbol);
+      printedChars++;
+    }
+    if ((qComma > knownMode) || (printedChars > u8x8.getCols() - 2))
+      break;
+  }
+  // Fourth row with palette name
+  u8x8.setCursor(2, 3);
+  qComma = 0;
+  insideQuotes = false;
+  printedChars = 0;
+  // Looking for palette name in JSON.
+  for (size_t i = 0; i < strlen_P(JSON_palette_names); i++) {
+    singleJsonSymbol = pgm_read_byte_near(JSON_palette_names + i);
+    switch (singleJsonSymbol) {
+    case '"':
+      insideQuotes = !insideQuotes;
+      break;
+    case '[':
+    case ']':
+      break;
+    case ',':
+      qComma++;
+    default:
+      if (!insideQuotes || (qComma != knownPalette))
+        break;
+      u8x8.print(singleJsonSymbol);
+      printedChars++;
+    }
+    if ((qComma > knownMode) || (printedChars > u8x8.getCols() - 2))
+      break;
+  }
+
+  u8x8.setFont(u8x8_font_open_iconic_embedded_1x1);
+  u8x8.drawGlyph(0, 0, 80); // wifi icon
+  u8x8.drawGlyph(0, 1, 68); // home icon
+  u8x8.setFont(u8x8_font_open_iconic_weather_2x2);
+  u8x8.drawGlyph(0, 2, 66 + (bri > 0 ? 3 : 0)); // sun/moon icon
+}