Implement a 'clean' command that removes everything created by redo

[Tharre]

redo saves hashes about everything it builds already in .redo/, so we might as well provide a clean command to save people from writing their own (often outdated) clean.do.

[Tharre]

It may even be a better idea to create a command that lists everything that was built by redo. That way it's more general, and the user has more power over what is being deleted.