passing context to GraphQLView no more supported?

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:

```python
# [...]

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](https://github.com/graphql-python/flask-graphql/issues/52#issuecomment-412773200)**

Comece pelo exemplo de contexto de GraphQLView no README e pelo ponto de entrada de GraphQLView; em seguida, compare o comportamento da versão 1.4.1 com a solução alternativa não documentada no comentário vinculado. A tarefa estará concluída quando for confirmada uma forma compatível de passar a sessão do banco de dados no momento da execução e o README ou o comportamento for atualizado de acordo.

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