Add '--installed' flag to `deps` command

I added the flag `--installed` to the 'brew deps' command. Executing the command `brew deps --installed` will only show the dependencies for the installed forumas. This flag overrides the `--all` flag.

This also has the nice side effect of allowing the `graph` command to also accept the `--installed` flag, so `brew graph --installed` will give you a nice Graphviz graph of the formulas installed on your system. It can be an alternative to `brew leaves` for understanding what packages can safely be removed.