Support for dual and triple usb serial

[maitredede]

Hello,

Teensy board can have multiple serial ports on USB, and the lib only supports single serial (default) mode.

When using single serial mode, the const USB_SERIAL is defined. For dual serial, only the const USB_DUAL_SERIAL is defined, and for triple mode USB_TRIPLE_SERIAL. So when using non-default mode, the disable/enableDriver functions are empty.

I tried to update the lib to support other modes, but I don't know how are called the enable/disableDriver functions, in all the lib, if it make a conflict...

I would be pleased if someone can give a try : https://github.com/maitredede/Snooze/tree/teensy36-multiserial