Feature/support branches

This PR adds in the 'support branches' feature:

* Support branches are for maintaining old versions of your app whilst develop & master are tracking the latest release
* Support branches are created from the master branch
* You can make releases from a support branch; when finished, they get merged back into the support branch
* You can make hotfixes from a support branch; when finished, they get merged into both the support branch and develop

This PR also includes:

* a bugfix in 'git hf init' for dealing with no origin / multiple origins (which I needed for testing this PR), 
* uses more of the common code in hubflow-common (made it easier to ensure feature, support, release and hotfix commands behave the same way)
* fixes for typos and summary messages that I spotted whilst adding support for support branches

It's an invasive change, so use with caution for now.