Support for Other GitHub-like Remotes

[allenh1]

Inspired by ros/ros-overlay#30

As we sit currently, rosdistro does not support things like GitLab and BitBucket with respect to fetching package.xml files.

[allenh1]

@dirk-thomas I'll work on this tomorrow.

[mikepurvis]

I'd be pleased to see this worked on. I contributed the manifest provider for BitBucket, but we've now switched to an on-prem GitLab, so having rosdistro cache updates be fast again would be great.