Add parsing hints for GitLab subgroups

[tjtelan]

https://about.gitlab.com/solutions/subgroups/

The main points I've discovered:

• subgroups can roll 20 levels deep
• The last segment of the URL seems to refer to the repo name

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Due to #10 this identification should be opt-in at parsing time.

[tjtelan]

The more I look into this, I've realized that subgroups and orgs are the same thing. It just happens that subgroups are used to define orgs. So in that case, I'll collapse #11 into this issue.