Configurable defaults for sbpy-ephem

[mkelley]

This is a request for

• a new feature
• an enhancement to existing sbpy functionality
• somethings else: [explain here]

The requested changes will be implemented by

• me
• the sbpy developers

High-level concept
Would be super nice to have configurable defaults for the sbpy-ephem script. For example, I generally like to see ephemeris uncertainties, but don't want to type --quantities=1,3,9,19,20,23,24,27,33,37 all the time (and yes I did make this a bash alias).