Support GitFlow AVH Edition

[pdfowler]

We've come to rely on the gitflow-avh implementation of the GitFlow branching model, as have many other developers. I've found the flexibility of having additional branch prefixes to be very helpful. Specifically, we have bugfix and refactor.

Rather than using the standard three GitFlow prefixes, it would be great if Fork read the [gitflow "prefix"] node of the .git/config file from each repo.

ie:

[gitflow "prefix"]
    bugfix = bugfix/
    feature = feature/
    release = release/
    hotfix = hotfix/
    support = support/
        refactor = refactor/

would add additional options in this menu:

[lksoft]

This would be awesome!