Feature Request: ISR-driven Example

I'm trying to use this library but I want the keypress handler to be ISR-driven, rather than by polling.

I have adapted the example code, and added a call to attachInterrupt(..) in the setup function. This causes a compile error:

WInterrupts.c.o (symbol from plugin): In function `attachInterrupt':

(.text+0x0): multiple definition of `__vector_1'

/var/folders/st/18vtghg95f7g8wmtqcdsj3x00000gn/T/arduino_build_207015/libraries/Pro_Trinket_USB_Keyboard_Library/usbdrvasm_includer.S.o:/Users/j/Documents/Arduino/libraries/Pro_Trinket_USB_Keyboard_Library/usbdrv/usbdrvasm16.inc:35: first defined here

The code triggering this, added to my setup function looks like:
`attachInterrupt(digitalPinToInterrupt(PIN_BUTTON_STRING), keyPressISR, FALLING);`

My implementation attempt aside, I would quite appreciate an example of using this excellent library, with the key presses being handled by an ISR. That'd be most excellent to offer.

Start with the adapted example's setup function and the attachInterrupt call, then inspect the compile-error references to WInterrupts.c.o and usbdrv/usbdrvasm16.inc. Determine the interaction between the requested keypress ISR and the library's existing interrupt handling. Done means a documented ISR-driven example that builds without the multiple-definition error.

Written by the indexing model from the issue text.