set default engine options while using SQLALCHEMY_BINDS

When engine options is not specified in <code>SQLALCHEMY_BINDS</code>, this engine will lack options with the bind. Which may cause some unexpected errors such as database disconnect error. <pre> SQLALCHEMY_BINDS = {"main": "mysql+pymysql://root:root@localhost:3306/dbname"} SQLALCHEMY_ENGINE_OPTIONS = {"pool_pre_ping": True} </pre> For example, if someone use Flask-SQLalchemy like this, he will think <code>SQLALCHEMY_ENGINE_OPTIONS</code> will make effect in <code>SQLALCHEMY_BINDS</code>. But that's not the case. So I suggest use <code>SQLALCHEMY_ENGINE_OPTIONS</code> as default engine options specifically when engine options is not specified in <code>SQLALCHEMY_BINDS</code>. So the code above will be same as <pre> SQLALCHEMY_BINDS = { "main": { "url": "mysql+pymysql://root:root@localhost:3306/dbname", "pool_pre_ping": True } } </pre>