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)**

先從 README 中的 GraphQLView context 範例和 GraphQLView 進入點開始，然後將 1.4.1 的行為與連結留言中的未記錄 workaround 進行比較。完成的標準是確認一種受支援、可在執行時傳遞資料庫 session 的方式，並相應更新 README 或行為。

由索引模型根據 Issue 內容生成。