Add process dependencies

[brentdodell]

It would be nice if you could make one process dependent on another so that:

1. If you try to start a process with dependencies, it ensures the dependencies are started first
2. And maybe if you restart a dependency, it restarts all processes that are dependent on it (this could be configurable?)

[kgrz]

Interesting. Stretching this a little further, won't we be looking at something like supervisord at that point? :) Also, trying to imagine what'd happen if one of the child processes exists; what should be the handling in that case.