Support nested templates

[Josh Manders]

Right now a single repo can only hold one template, and the whole repo IS the template. Lets allow a repo to have multiple templates.

For example if you run groundwork foo/bar my-app then it will fetch and scaffold out foo/bar/tree/${DEFAULT_BRANCH}.

If you run groundwork foo/bar/baz my-app then it will fetch and scaffold out foo/bar/tree/${DEFAULT_TEMPLATE}/baz

If you run groundwork foo/bar/baz#qux my-app then it will fetch and scaffold out foo/bar/tree/qux/baz.

This will then allow us to ship "official" templates, as we can default single node templates in the groundwork repo, and you can then do groundwork next-starter my-app and that will fetch and scaffold out aniftyco/groundwork/tree/master/templates/next-starter.