New usermod: MQTT switches.

This user mod adds a function to toggle output pins via MQTT.

usermods/mqtt_switch_v2/README.md

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+# MQTT controllable switches
+This usermod allows controlling switches (e.g. relays) via MQTT.
+
+## Usermod installation
+
+1. Copy the file `usermod_mqtt_switch.h` to the `wled00` directory.
+2. Register the usermod by adding `#include "usermod_mqtt_switch.h"` in the top and `registerUsermod(new UsermodMqttSwitch());` in the bottom of `usermods_list.cpp`.
+
+
+Example `usermods_list.cpp`:
+
+```
+#include "wled.h"
+#include "usermod_mqtt_switch.h"
+
+void registerUsermods()
+{
+  usermods.add(new UsermodMqttSwitch());
+}
+```
+
+## Define pins
+Add a define for MQTTSWITCHPINS to platformio_override.ini.
+The following example defines 3 switches connected to the GPIO pins 13, 0 and 2:
+
+```
+[env:livingroom]
+board = esp12e
+platform = ${common.platform_wled_default}
+board_build.ldscript = ${common.ldscript_4m1m}
+build_flags = ${common.build_flags_esp8266} 
+   -D LEDPIN=3
+   -D BTNPIN=4
+   -D RLYPIN=12
+   -D RLYMDE=1
+   -D STATUSPIN=15
+   -D MQTTSWITCHPINS="13, 0, 2"
+```
+   
+## MQTT topics
+This usermod listens on `[mqttDeviceTopic]/switch/0/set` (where 0 is replaced with the index of the switch) for commands. Anything starting with `ON` turns on the switch, everything else turns it off.
+Feedback about the current state is provided at `[mqttDeviceTopic]/switch/0/state`.
+
+### Home Assistant auto-discovery
+Auto-discovery information is automatically published and you shoudn't have to do anything to register the switches in Home Assistant.
+