Button event lib support positive pulse button.

[Sean shao]

I got a new pico board “Pico:ed“. A low level is generated when the button is pressed and a high level when it is released.

The Button event library can support this type of button in a more general way.
My current approach is reversing the logic of _readbutton method in the Button event lib.

Thanks

[John Maloney]

I've decided not to add this feature to the library since it is seldom needed. In the case of the pico:ed, you can just use A and B since those buttons are implemented by the pico:ed VM. In the rare cases where it actually is needed, one can edit the library to invert the button.. Hope that's okay...