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!