Plugin mechanism

[pcoccoli]

Currently, the psycopg2 package is required for postgresql support. However, if you're using sqlite3, you shouldn't need to install psycopg2. Using a plugin mechanism to separate out different "back ends" would solve this.

[pcoccoli]

While still not implemented, PostgreSQL support is now an "extra" which can be installed via firepit[postgres] or firepit[async-postgres]