Permit non github urls

Really wanted to use antigen for a company specific repo that was served over the local network via http and ssh. Unfortunately antigen was consistently rewriting the URLs to include https://github.com. Expanding the list of acceptable looking URLs fixes this issue.

Antigen is so good it shouldn't be restricted.

Don't really know a good way to handle the user@host type thing apart
from just looking for an @, which seems overly broad.