Add `add-contributor` command

[extsoft]

This command registers a new contributor (a fork) as an additional remote repository.
Sample command:

git remote add alexbeatnik git@github.com:alexbeatnik/dp-151.git

Desired signature:

usege: git elegant add-contributor <git URL>

This means that remote name should be automatically parsed from URLs like

• git@github.com:alexbeatnik/dp-151.git
• https://github.com/alexbeatnik/dp-151.git