Feature Request: Add support for thumbstick buttons

[kgsbowtie]

Basically implement the buttons on the thumbstick on controllers. Support for these controllers was evidently added in iOS 12.1.

I don't know enough about iOS programming to implement it properly, but I do know from poking around that you'd need to implement extendedGamepad.leftThumbstickButton and extendedGamepad.rightThumbstickButton in MFiGameController.swift.

[lbibass]

this has been addressed in this pr: #33