add options '--deps-up-to', '--deps-only'; '--exclude', add support for special keywords 'ALL', 'RPP'; implement reverse dependency walk

@tfoote @wjwwood Please review  - especially the logic of the algorithm how stuff is getting selected. I tried to be verbose enough in my inline comments that it should be possible to follow the general flow.

It would also be great if you can give it a try and run a couple of queries and check if they look reasonable (it requires the latest version of rosdistro from source).