Inherit configs/discriminators from parent class + add option for discriminator to match current class

[kdkavanagh]

Fixes #321

I initially had an impl (the first commit on this branch) that solely fixes the config-from-base-class issue, but in implementing that, I discovered the restriction against matching the current class w/ a discriminator. Adding support for that seems useful (for my usecases, anyway), and reduces code duplication (as compared to the impl in my first commit)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.98%. Comparing base (c1820be) to head (970061d).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master     #322    +/-   ##
========================================
  Coverage   99.98%   99.98%            
========================================
  Files         101      101            
  Lines       11911    12062   +151     
========================================
+ Hits        11909    12060   +151     
  Misses          2        2            

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.