Add ability to add members to a repository

Problem to solve

I want to be able to add contributors by username to my repository as contributors/developers/maintainers/owners.

Proposal

A new sub-command glab repo members add <USERNAME> <PRIV_LEVEL> would make the most sense to me.

Further details

Easier to add people from the command line than from the web browser.

go-gitlab support

• https://github.com/xanzy/go-gitlab/blob/master/project_members.go