text: Title: Provide a way to configure the SA engine
  Body: Hi all,
  
  Unless I'm mis-reading the code, there is no way to provide engine creation options. One of them that appears with SA 1.2 is [pool_pre_ping](http://docs.sqlalchemy.org/en/latest/core/pooling.html#pool-disconnects-pessimistic). 
  
  I'm not sure I can provide extra parameters via flask-sqlalchemy parameters. Should I create the engine out of band?
  
  Thanks,