Allow setting feature gates for groups or projects

After gitlab-rails accepts a feature gate based on projects, through https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24184 support for passing this value through chatops was needed still.

--project= as flag was choosen, the flag is accepted for any position after the set keyword in the command.

A later iteration added --group= support, when the feature gate uses a group instead of a project.

In both cases the arguments are paths of the entity.

Closes #50 (closed)