Often, it's convenient to have a Runfile in a central place, like the root folder of a repository. I think it would be nice if run could search upwards to try find a Runfile if one is not found in the present directory, to make working in a subdirectory more convenient in that case.

Suggested workflow:

• if -r is found, then require that file to exist. Don't search any other path.
• if not, then search for Runfile in the current directory, and if it does not exist, keep searching upwards until a Runfile is found

We could also apply some other bounding here, like not continuing upwards if a VCS directory (.git, etc) is found, but perhaps that isn't really needed?