Support schema-less data files

[kevlarr]

Schemas aren't always known up front and might be specified by a connection string, so hardcoding them in a data file is unnecessarily limiting.

Some potential options might be..

1. Defaulting to schema-less and allowing qualifying the table name with schema

table1
  ..cols

public.table2
  ..cols

2. Optionally nest tables in an explicit schema

table1
  ..cols

public
  table2
    ..cols

[kevlarr]

This is actually blocked on #44