Support GitHub Enterprise repository links

Let companies or groups that run their own GitHub Enterprise installs be able to link their own repositories. That's the case with our company and we can't easily link the internal repositories.

I tried to solve that by adding a setting called `github_url`, which can point to the address of the local GitHub install. By default it is `https://github.com` and if that makes it into `0.3.0` it won't fail if you forgot to include it in the config.