Support Git tag prefixes

[andir]

Some projects (notably Emacs) have prefixes in their release tags (e.g. emacs-1.2.3) instead of just bare version numbers. We should support these kinds of tags.

There are two rough ideas that come to mind:

• Search through all tags and determine a common prefix (that doesn't represent a version)
• Let the user specify a prefix that should be stripped and that will be used as a filter.
  Some repositories carry more than one project that might be release independently.

[piegamesde]

At the moment we use a fuzzy SemVer parser. It will accept bare version numbers with a few additions to the official specification. It will also automatically strip a leading v to the version number.

I think we could try to strip *- from the tags and see if they parse after that. I'd like to not have users specify a prefix, and I'd like to call repositories with multiple (independently tagged) applications out of scope.