Accept JSON payload

[kaovilai]

https://ifttt.com/maker_webhooks/triggers/json_event

This would help minimize number of applets needed when IFTTT filters can be used to alter actions.

ie. copy paste a button and alter slightly the payload content such as brightness from 50% to 70% without creating a new ifttt applet for each brightness level.

[tobimori]

I'm happy to accept a PR for this, but I won't be able to add this to the plugin in the near future by myself.
Requirement for editing JSON would be monospace font + auto-beautify on save + validation on typing if possible.