Add support for prefixed version tags in scc

With this commit, we sould now be able to tag the repository using
semver's recommended vX.Y.Z-METADATA. `get_git_version` has been updated to
recognize this new format so that both `scc version` and `python setup.py sdist` should work fine.

To test this PR:
- check the unit tests pass
- create a local annotated tag e.g. `v0.5.0-rc1` and check `python scc/main.py version` and `python setup.py sdist` use the correct version

/cc @rleigh