Bean reference missing inherited properties

The [beandoc.py](https://github.com/internetarchive/heritrix3/blob/master/docs/_ext/beandoc.py) script used to generate the [bean reference](https://heritrix.readthedocs.io/en/latest/bean-reference.html) manual is currently unaware of properties inherited from superclasses. To improve completeness it should resolve and parse parent classes too.