Create fork of the active repository by subcommand
    
    Adds new command 'fork' that calls API method which forks active
    repository for the given (or active) user and creates a remote record
    (named after user) in git configuration.
    GitLab Personal Access Token have to be added to the config for
    proper functionality.
    
    Signed-off-by: Ondrej Nosek <onosek@redhat.com>