Add Collaborator To a Repo (with specific permissions)

I would love for this library to wrap the following end point:

`PUT /repos/:owner/:repo/collaborators/:username`

Link to documentation: https://developer.github.com/v3/repos/collaborators/#add-user-as-a-collaborator

Would be happy to submit a PR for it.

Start by reviewing the library's existing GitHub API wrapper patterns, then use PUT /repos/:owner/:repo/collaborators/:username and the linked GitHub documentation as the entry point. Done means the public interface wraps adding a collaborator while supporting the endpoint's specific permissions, with behavior checked using the repository's existing verification approach.

Written by the indexing model from the issue text.