Add reinstall command

Right now there's no real automated way to update casks except than removing and reinstalling items.  This patch acts as a stopgap measure to make manual updates slightly less painful, to be possibly deprecated once automated updates are in place.

This can probably be replaced by a shell script/shell alias, but maybe it might still be useful if integrated in the brew-cask script.  The patch didn't touch the changelog, as I'm not sure what's the policy for that.