Add a "gitflow hotfix checkout" subcommand

[jkankiewicz]

Being that there could be multiple support branches existing simultaneously along with the main branch, the gitflow command should enable one to switch from working on a bugfix/feature/release branch to a hotfix branch or from a hotfix branch parented by one support branch to a hotfix branch parented by a different support branch (or the main branch).

$ git flow version
1.11.0 (AVH Edition)
$ git flow hotfix checkout ...
Unknown subcommand: 'checkout'
usage: git flow hotfix [list]
   or: git flow hotfix start
   or: git flow hotfix finish
   or: git flow hotfix publish
   or: git flow hotfix delete

    Manage your hotfix branches.

    For more specific help type the command followed by --help