Allow to specify dialect/driver when calling get_connection_url #140

With latest SQLAlchemy 1.4 rolling out support for asyncio it would be great to support that in some way.

In order to use asyncio one needs to use a corresponding driver which is specified in the connection string, e.g.:

postgresql+asyncpg://scott:tiger@localhost/test

But in PostgresContainer it's currently hardcoded to:

postgresql+psycopg2

Don't know what the best solution could be:

• Pass it as a constructor argument - but in this case there would be a limitation that get_connection_url will return fixed connection string, which this might not be a huge issue realistically, but feels odd, since container doesn't really care how we want to connect to it
• Pass an argument to get_connection_url, but this messes up with an API provided by DbContainer and then other implementation should probably support this as well or at least ignore unknown parameters.