Push options: print help information

Problem to solve

While push options is a very powerful feature, it can be opaque and hard to use even for folks who are relatively versed with GitLab and the command line.

Proposal

Add a push option that doesn't actually complete the push, but instead sends back an explanation of using it to the command line like:

git push -o gitlab.help

or similar.

I'd guess returning like the 5 most common options and then the URL to the help page would be stellar