Get the release tag for jade

Resolved https://github.com/marionettejs/marionettejs.com/issues/25

Allow for cli -TAG=v.X.X.X to override the latest tag.
Cache tag on a global

Possible refactors:
- Cache sorted tags
- Split up tag retrieval from the checkout and only checkout when necessary
- gitty util for tag sorting functions shared between gitty and compile-docs