mr_list: substitute --mine by --author
mr_list: substitute --mine by --author

The `--mine` option was a short hand for `--assignee <lab.User()>`, which
was causing some confusion for new users since it gives the feeling of
"possession": the user owns/authors that MR. This patch removes the `--mine`
option, since it easily replicated with `--assignee` and adds the `--author`
option, so the user can filter the MR he authors.

Signed-off-by: Bruno Meneguele <82c7731bc2625bcbab88bc571aee6631ba3f66f3@redhat.com>

zaquestion/lab,zaquestion/lab