Dependency overview flags: --of-any and --of-none

*Rationale*: in a system management situation, have a quick overview of
what's been pulled in as a dependency and what is a potential candidate
for removal. To that end, I added two modifier flags to `brew deps`:

`--of-any`:  lists all formulae that are a dependency of any formula*.
`--of-none`: lists all formulae that are not a dependency of any formula*.

*) 'any formula' actually means formulae to which we apply the flags,
e.g `--installed` or `--all`.

`--of-any` respects `-n` if given, while it makes no sense for `--of-none`.