Add repository search

I am not the happiest with how the endpoint behaves, but I have an idea.

I'm thinking of adding to the dictionary returned in the instance where `text_match is False`. That will be a separate list of `Repository` objects. That said, the search results can be paginated and I'm not sure how to present that information to the users.

Another issue is that the repositories are returned with a score (ostensibly to indicate how well they rank in the search).

TODO:
- [x] Add method to the public api, i.e., `search_repositories` with an admonishment about the severely low rate-limit
- [x] Make sure doc-string(s) are up-to-snuff