Validate assignments to rcParams-like dictionaries

[lukelbd]

The rcParamsShort and rcParamsCustom dictionaries in rctools.py (used to power the rc_configurator) should have restricted keys and automatic entry validation, like rcParams. This can be done using the existing public matplotlib API and the RcParams class. It's a minor issue, maybe post-v1.

[lukelbd]

This change is ready to be added after the #91 merge

It probably does need to be included in v1 because otherwise incorrect entries in .proplotrc or assignments to plot.rc are extremely difficult to debug. We can borrow the various validate_* fucntions from matplotlib/rcsetup.py, and might want to also design a new "units" validator (inches, centimeters, etc.) and refactor the units function to use this validator.