More robust commit-label parsing, and Unicode support in labels

Note: at the moment, the embedded regex doesn't separately capture the label-parameter, from the label itself. Theoretically, that can be left to the regex engine, saving manual splitting thereafter.

I also added a plug / explanation for `.gitlabels`; but I've intentionally kept that in a separate commit, so feel free to discard that one if you think it superfluous or inappropriate!

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Also: I'm not much of a Pythonista. Not that you wouldn't already, but you should double-check that my changes execute as intended. (I've verified that Python 2.x's `re` supports the lookahead I'm using there, FYI.)