Add the possibility to ignore branches based on a regex.

  - the regex is passed to -i or --ignore-branch
  - the safest is to pass it single-quoted, in order not to have it on
    interpreted by the shell, and to escape it where appropriate
Gracefully handle missing branch on remotes
Oops, since we're using git -r, remote branches are not prefixed by 'remotes/'
Branch ignore: yes, -i take an argument. Or more coffee.
Merge pull request #1 from sdelafond/master

Various improvements, feature added by @sdelafond