Interaction Fixes and other cleanup

@joe-goullaud 

- Disable interaction with main VC while open by default.
- Allow delegate to dictate if reveal pan should begin
- Tap gesture to close
- Flag to enable peeking, disabled by default
- Refactor reveal offset so it makes more sense (to me at least).
   - Maybe I misunderstood what it was for. Please let me know if I did.
- Add proper child VC containment.