Add collision handling

[michaeloryl]

Provide support for running code when there is a collision. Perhaps call a defined gosub?

Not sure how to handle the current execution queue after the collision is handled, though. Carry on from where it was? Seems unlikely to be useful for anything other than flashing a color or setting a few status variable values.

Perhaps if the collision handler was associated with a block of commands, it might be more useful. Then the block could reset or be considered complete after the handler runs. Not sure.