Support site names other than github.com (i.e. for GitHub Enterprise)

[synthead]

This plugin works great for all github.com repos, but it does not work with GitHub hosted on other site names, like company-hosted GitHub Enterprise instances.

For example, with GitHub hosted at github.mycompany.com, with an org name of example-org and a repo name of example-repo, :ToGithub produces this URL:

https://github.com///blob/caf758b2dbe03480dd340dd339379ca86797e6b6/README.md#L11-L27

...when this URL is expected:

https://github.mycompany.com/example-org/example-repo/blob/caf758b2dbe03480dd340dd339379ca86797e6b6/README.md#L11-L27