Sqlite support

We need to configure a few things to get everything working. As it stands, switching to SQLite 3 causes a few configuration options that default to calling `Rails.#{method}` to blow up.

See: https://github.com/rails/rails/blob/08754f12e65a9ec79633a605e986d0f1ffa4b251/activerecord/lib/active_record/tasks/database_tasks.rb#L27-L34

One thing that's missing: `DatabaseTasks.seed_loader` (an object that responds to `load_seed`). Without it, I don't think we have proper `db:seed` support (but I may be wrong).

This PR additionally changes the default from PostgreSQL to SQLite 3. I think it's the better default when you just want to hack on an idea. People'll switch to Postgres as soon as they need to, anyway.