brew-test-bot: change default email subject.
brew-cleanup-installed: add new external command.

brew-cleanup-installed: uninstall all non-whitelisted Homebrew formulae.

Useful for maintainers/testers who regularly install lots of formulae
they don't actually use.

Populate ~/.brew-cleanup-installed with the formulae you want to keep
installed. All others will be uninstalled when brew-cleanup-installed is run.