Allow for git repositories

[thecb4]

As a user I want to be able to pass a git repository instead of a template.yml file so that I can maintain my own templates and make use of community templates.

use:

$ mkdir HappyDance && cd HappyDance
$ genesis generate https://github.com/thecb4/GenesisTemplate

1. It should allow for http || https || git protocols
2. It should download to a common folder (~/.genesis)