Bring 'else' back? #89

There were several commenters on today's revealing of the PEP on python-dev who want an else clause instead of case _: to enter final catch-all block. It's not terrible. If we do this I propose it should be indented with the case clauses:

match t:
    case P1():
        ...
    case P2():
        ...
    else:
        ...

I'm trying to hold this off a little longer, but it may be an easy thing to give.