Support for GitHub Enterprise

By using a `GitHub-URL` header and an `Authorization` header, you can
now pull down gists using GitHub Enterprise:

```sh
curl -H "GitHub-URL: https://github.example.com/api/v3" -H \
"Authorization: token $TOKEN" http://runmygi.st/username/1234567890abcdef
```

Signed-off-by: David Celis <me@davidcel.is>