add engine kwargs

[comhar]

fixes #27

This pr adds an engine_kws param to the Database class. For databases like postgres this makes it easy to adjust parameters like the connection pool size, max_overflow or set the application name which makes database performance troubleshooting, fine-tuning much easier.

db = Database("postgresql://...", engine_kws={"connect_args": {"application_name": "main_app"}, "pool_size": 2, "max_overflow": 3})

[jph00]

good idea :)