Add new optional config parameter.

Add new parameter to configure the minimum approach time to use when
skip_planning = true.

If we time-parameterize trajectories using MoveIt's built-in methods, we'd
be able to get rid of this additional parameter, but in the meantime, it
addresses an important issue.
Add default values and units to sample config.
Merge pull request #42 from pal-robotics/min-approach-dur

Add new optional config parameter.