Add optional custom git tag prefix, version 2

Based on #11, but maintaining backwards compatibility on the `version_released?` method, and adding some extra documentation of the existing code.
Technically, as it turns out, that’s still valid Ruby (according to a note tucked away on [page 48 of The Ruby Programming Language](http://books.google.co.uk/books?id=rbY5mz-_VdQC&lpg=PR1&pg=PA48). This isn’t something I knew about before today.

I’ve added some braces in d2d62a9; it’s less confusing that way.