Add a new feature: allow to specify and use a third party repository
for which there is no configuration, so one can do:
slacker repo=URL [install|...] repo[:pkg1,pkg2,..].
Turns off GPG automatically if it finds no GPG-KEY.
Other arguments are not allowed, unfortunately, so one can only use
the repository specified by using this feature.
This was inspired by slackpkg+. Thanks!