Add 'strip_branchname_prefix' option for git

Gitolite supports [personal branches](http://gitolite.com/gitolite/user.html#pers), which are a convenient way to allow a large group of developers push access to any number of branches within their own dedicated namespace. Personal branches require a fixed prefix on the branch name (e.g. "personal/". It works really nicely as a way to deliver dynamic Puppet environments, but it does add some noise to the naming - given enough development streams, you wind up with lots and lots of Puppet environments called `personal_<something>`

This commit provides a 'strip_branchname_prefix' option, to allow you to strip a fixed string from the start of a branch name before r10k goes and does its stuff.

(I'm not sure how widely used personal branches are for Puppet, and therefore how many other r10k users might want something like this - but we needed this to migrate to r10k from our custom post-hook.)