passing context to GraphQLView no more supported? #52

Hi and thanks for the geat job.

With 1.4.1 version I used to pass my DB session via the context option of the GraphQLView as still mentioned in the README (which should be updated).

Below is an exemple of what I was doing:

# [...]

db_session = scoped_session(
    sessionmaker(autocommit=False, autoflush=False, bind=engine))

# [...]

app.add_url_rule(
    '/graphql', view_func=GraphQLView.as_view(
        'graphql', schema=SCHEMA, graphiql=True,
        context={'session': db_session}))

# [...]

It seems this is no more possible and I can't figure out how to pass the DB session to the GraphQLView. Any information and/or updated doc would be nice.

Any solution to continue passing DB session via the GraphQLView (or by any other way allowing to pass it at execution time) would be perfect.

EDIT: for an undocumented but working solution see @cockscomb 's comment