brew reinstall: Is able to work for all formulae

... and not just installed ones. Of course, strictly speaking,
reinstalling not-yet-installed formulae makes semantically little
sense, but the big win is that we can tell people (after we have
resolved an issue) to `brew reinstall <formula>` and even if a user
has removed that formula in the meantime, reinstall will do the right
thing. Basically adding --force to uninstall. I think this makes
reinstall more robust.