Support for additional engine parameters

[braybaud]

I am trying to extend the SQLite timeout parameter.
To do so, I need to modify the create_engine parameters.

It would be nice to add an connect_args to the get_db function allowing to pass:

db = get_db(application, ..., connect_args={'timeout': 15})

which will add this to the create_engine.

We could also think making it generic and just pass the **kwargs of the get_db downstream to the create_engine