Integrating support for graphene-sqlalchemy-filter into code base

As there is limited filtering with [graphene-sqlalchemy](https://github.com/graphql-python/graphene-sqlalchemy), I have been using [graphene-sqlalchemy-filter](https://github.com/art1415926535/graphene-sqlalchemy-filter) which has many stock filters included. Unfortunately the developer seems to have lost interest in the code base and a critical PR to add support for Graphene 3.0 has been unmerged for six months.

- https://github.com/art1415926535/graphene-sqlalchemy-filter/pull/63

I have tried contacting the developer a few times without response.

As the filtering code for the add-on is contained in the `graphene-sqlalchemy-filter` package's two files, I propose merging this code into `graphene-sqlalchemy`.

Would this be OK?