Add support for Digispark boards

[veprolet]

This pull request adds minimal support for the Digispark model A board from Digistump and in general allows for boards with the micronucleus bootloader to be added.

This attempts to supersede both #367 (add the board api) and #364 (add micronucleus support to ravedude), as they seem to have stalled. If deemed appropriate, this PR can be reduced in scope and #367 still used.

Few notes about the implementation:

• As far as I'm aware, model B of Digispark is no longer manufactured and model A is the canonical version people refer to when using "digispark", so I made no attempt at including the model specification in the digispark identificator.
• Flash verification is disabled, as the micronucleus bootloader doesn't support it. No warning is issued to the user about this.
• Waiting for board reset is delegated to avrdude instead of the usual "Press enter after you reset the board", as this would make programming boards in the default bootloader configuration almost impossible. It's possible to configure timeout for this operation, but this PR leaves it indefinite.