Allow selection for custom remote names on github

For example on a `$ git remote -v`

```
gt      https://github.com/ocaml/ocaml-re.git (fetch)
gt      https://github.com/ocaml/ocaml-re.git (push)
origin  https://github.com/rgrinberg/ocaml-re.git (fetch)
origin  https://github.com/rgrinberg/ocaml-re.git (push)
```

I'd like to use `gt` and `origin` as well as usernames for repo selections.
