Add Savannah as a supported forge for quick commands

[davep]

While I personally seldom find myself looking on there, it might be nice to add support for savannah to the list of "forge quick commands". Given that it seems to break down into gnu and nongnu variations I suspect those might be the commands to go with.

TODO:

• Find a good example of a GNU project that actually has a README.md.
• Find a good example of a non-GNU project that actually has a README.md.