Add support for removing and getting collaborators

It might be useful to use this module for more than just adding collaborators. I'm not sure if you we should allow for removing to be used in conjunction with `addToAll` and `addToSources` though. Could just take a user and an array of repos.

Also, a `.get` or `.check` method should be added to get all collaborators of a repo. Should it take more than one repo though?

Start by reviewing the existing collaborator module and the addToAll and addToSources entry points. Clarify whether removal should compose with those operations and whether get/check accepts one repo or multiple repos. Done means the supported method signatures and behavior are explicitly decided and implemented with coverage for the chosen cases.

Written by the indexing model from the issue text.