Gitty caching and tag util

Resolved https://github.com/marionettejs/marionettejs.com/issues/25
Vastly improves https://github.com/marionettejs/marionettejs.com/pull/38

Allow for cli -TAG=v.X.X.X to override the latest tag.  
Otherwise gets and caches the latest tag on the repo.

Not thrilled with setting the exact same options for `gitty:checkoutTag` and `gitty:releaseTag` Is there a better way?
Wasn't certain of a best location for utils.
I don't think there's a way to utilize a promise for the jade data, so `git:releaseTag` must come before `jade` or you'll get `v.X.X.X` in the HTML.