Rethink public API in light of Adapters

Right now, the programmatic and CLI API are hard-coded to automatically use the Postgres adapter. They should support any adapter.

What I'm thinking is that I'll need a public config file, maybe `.plsrc`, to configure which adapter to use. Then, the adapter will need to be in `node_modules` and have some methods or files defined.

Start by tracing where the programmatic and CLI APIs hard-code the Postgres adapter. Define the public configuration approach and the adapter methods or files required, then verify that both APIs can select an adapter installed in node_modules.

Written by the indexing model from the issue text.