--url and --token are now url and token

Update error messages to reflect the fact that --url and --token are now
action that must be specified with url and token.

    You should set token by 'gitlab --token abcdefghij123456'

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #1 from dachary/master

--url and --token are now url and token