Download references to tags as well as branches

[hews]

Eg:

git grab https://github.com/hews/git-grab/releases/tag/v0.2.1(/*)
=> svn https://github.com/hews/git-grab/tags/v0.2.1(/*)/

Include directions and pointers: that this only works with references to branches, tags and master (not commits), for example, and how the links should be formatted.

[hews]

Should also work with:

https://github.com/abc/xing-backend/tree/v0.0.14

… but that means, for all /trees, trying for /branches and then /tags if the first fails. Much harder.