Use this method to register/assign a custom trigger listener.
This callback listener is fired when the IFTTTMakerChannelTrigger
receives a GPIO pin change that invokes the trigger. This listener
provides the consumer an opportunity to be notified of the trigger event,
to optionally abort the triggered event before sending data to the IFTTT
Maker Channel API, or an opportunity to override any data payload values
before transmitting to the IFTTT Maker Channel API.