[MH-Z19] Add 'ABC disable' option to web config (letscontrolit#472)

* [MH-Z19] Add 'ABC disable' option to web config

As discussed in letscontrolit#466.
The default setting is the current behavior.
When changing the setting, it will be applied as soon as there is a stable reading.

* [MH-Z19] Read settings first before determine must apply flag.

src/_P049_MHZ19.ino

@@ -108,11 +108,11 @@ boolean Plugin_049(byte function, struct EventStruct *event, String& string)
 
     case PLUGIN_INIT:
       {
+        Plugin_049_ABC_Disable = Settings.TaskDevicePluginConfig[event->TaskIndex][0] == ABC_disabled;
         if (Plugin_049_ABC_Disable) {
           // No guarantee the correct state is active on the sensor after reboot.
           Plugin_049_ABC_MustApply = true;
         }
-        Plugin_049_ABC_Disable = Settings.TaskDevicePluginConfig[event->TaskIndex][0] == ABC_disabled;
         Plugin_049_SoftSerial = new SoftwareSerial(Settings.TaskDevicePin1[event->TaskIndex], Settings.TaskDevicePin2[event->TaskIndex]);
         Plugin_049_SoftSerial->begin(9600);
         addLog(LOG_LEVEL_INFO, F("MHZ19: Init OK "));