Add support for Github enterprise repositories

[d1egoaz]

Add support for Github enterprise repositories as hostnames are different than github.com.

This is my first time writing LISP code, so I hope I'm not doing something crazy here.

[diego-alvarez-hs]

Ping @osener

[ozanmakes]

@diego-alvarez-hs sorry for taking so long to review this. I see you're taking the full url from remote.origin.url but I think more often than not non-github values will point to something like Bitbucket rather than a Github Enterprise repo.

What do you think about introducing a local variable such as github-browse-file--github-enterprise-root and using it instead of github.com if it's non-nil?

/cc @dgtized, @wjlroe, and @waymondo as you guys are probably more familiar with this package than I am at this point, and hopefully have experience with Github Enterprise :)

[d12frosted]

Also, relying on remote.origin.url is not really good, because origin is optional name.

[slackorama]

I took a stab at adding support for github enterprise and bitbucket...as well as the non-origin remotes.

[jjrussell]

I think I just wrote almost the same thing. Should have checked for existing PRs I guess #facepalm
#26