What about falling back to `${ATOM_REPOS_HOME} || ${HOME}/github`, like `apm develop` does?

Actually, it would be nice if `apm develop`'s default path and `core.projectHome` were linked somehow, too... but I'm not sure how to do that, since Atom's Settings page can't edit the environment variable in a permanent way. Maybe changing `apm` to read it from `.atom`?