Create SQLAlchemyConnectionField through a factory method

[dennylab]

Overriding the constructor-method gives the user the possibility to enhance the standard implementation of connection fields. (If you want to add filtering/sorting for example)

[coveralls]

Coverage increased (+0.2%) to 91.339% when pulling 073dc55 on labsolutionlu:ConnectionFactory into 030e5e1 on graphql-python:master.

[syrusakbary]

Thanks for contributing!

[palisadoes]

My organization has been using graphene-sqlalchemy-filter for filtering, however the repo has become inactive. Would anyone on this thread be interested in integrating the graphene-sqlalchemy-filter features into graphene-sqlalchemy as a sponsored activity?