Add local.branch.current.tag to get the tag if any

First, thanks for this so very useful Grunt plugin! :-)

Then, here is a PR that adds the information `local.branch.current.tag` if the branch is a detached HEAD and if there is a tag for the current state.

Rationale: In all the projects we use we discovered that we need the tag to display it as the version number. Other people have the same need, see http://stackoverflow.com/questions/18659425/get-git-current-branch-tag-name