Release 0.4.0

Features

• Adding max_terms_per_where option allowing to limit the number of
  terms in generated SQL in a call to addN

Fixes

• Restoring the setuptools RDFLib store plugin entrypoint so users no
  longer need to call registerplugins in most cases

• Making addN, add, bind not error out when there's a conflicting
  entry

Maintenance

• Removing uses of rdflib.py3compat

• Adding configs for generating documentation

• Minor documentation cleanup

• Supporting Python 3.6 and 3.7