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
To be fair here, in my ignorance I did float this idea first in gitter:

> paulfalgout 4 hours ago 
so I think once you get the latest tag we should just set a global or a config or something, so it’s only done the once
samccone 4 hours ago 
yeah that would work
maybe
hahah

:open_mouth: