Support for co-authors

[nebbles]

To support co-authors on GitHub, the following trailer needs to be added to the commit

Co-authored-by: Firstname Lastname <some.one@example.foo>

• There must be at least one blank line between this line and the previous lines of the commit (two is recommended)

Additional features:

• It would be useful if gitcommit can cache these authors and save them in a global config since typing them out every time would become a pain.
• It would be very useful if you could type in a GitHub username e.g. nebbles@gh and gitcommit automatically detects and attempts to resolve the public email from the GitHub profile page (and possibly full name?)

[nebbles]

https://help.github.com/en/github/committing-changes-to-your-project/creating-a-commit-with-multiple-authors

[nebbles]

As previously mentioned, the exact specification on how to add a co-author as a 'trailer' to the commit message can be found here:

https://help.github.com/en/github/committing-changes-to-your-project/creating-a-commit-with-multiple-authors

Support for coauthors was added to the GitHub desktop app. In this app, the authors are resolved through the API via a mentionable user.

The source code for getting mentionable for a particular repository:

https://github.com/desktop/desktop/blob/ab28d00115081f70bf773d36216dad4ab15be10a/app/src/lib/api.ts#L962

There is also an API endpoint for getting the public email addresses of a particular GitHub user:

https://developer.github.com/v3/users/emails/#list-public-email-addresses-for-a-user

This PR for Atom, discusses adding mentionable users with the GitHub GraphQL API

https://github.com/atom/github/pull/1476/files#diff-961f5e053a04f04fd4a96b42712e4dc4R133

The line of interest where the GraphQL request is made, can be found here

https://github.com/atom/github/blob/dd9647641beb263253c5678b888b7964c5c675cb/lib/models/user-store.js#L140

The documentation for the repository object on the GitHub GraphQL API can be found here

https://developer.github.com/v4/object/repository/

The Atom repository may be of interest as they also implemented using local git history in order to determine mentionable git users.