Add a pifacedigital_wait_for_input2() that returns wether or not interrupts were triggered

Hello,

This PR adds a `pifacedigital_wait_for_input2()` function that allows the caller to know wether or not interrupts were triggered.

It uses the return value from `mcp23s17_wait_for_interrupt()` so I believe the behavior is correct. If it's indeed OK, can you please merge this PR?

Thanks,