mr_list: Add --reviewer option
mr_list: Add --reviewer option

Add an option, similar to the --author and --assignee search options, that
allows users to search on the GitLab Reviewer field.

Add a --reviewer option to return the list of all merge requests for
which the specified user is listed as a GitLab Reviewer.

Signed-off-by: Prarit Bhargava <fe4655370dc87a32329d591d6e8472329333dfad@redhat.com>

zaquestion/lab,zaquestion/lab