Add support for the endpoint for searching users

[bilal-fazlani]

Given public email address of a commit author, I want to be able to fetch user information such as avatar url. The current getUser api accepts only username which I don't have. I need to be able to search users by their email address. Github seems to support that https://help.github.com/en/github/searching-for-information-on-github/searching-users
I was wondering if that support is present in this library. I could not find it in the documentation.

[BenFradet]

It's not supported at the moment, we would have to support this endpoint: https://developer.github.com/v3/search/#search-users.