Mark all required fields for config. Throw if no input_* field specified

- Add custom validator to throw if neither input_filter or
  input_<files|directories> are specified

- Set field expecting paths to type Path

- Now that default_config isn't used in code. We can update
  fields in rawconfig to specify whether they're required or not.
  This lets pydantic validate config file and throw appropriate error

tests/test_cli.py

@@ -40,5 +40,5 @@ def test_cli_config_from_file():
     assert actual_args.config_file == Path('tests/data/config.yml')
     assert actual_args.regenerate == True
     assert actual_args.config is not None
-    assert actual_args.config.content_type.org.input_files == ['~/first_from_config.org', '~/second_from_config.org']
+    assert actual_args.config.content_type.org.input_files == [Path('~/first_from_config.org'), Path('~/second_from_config.org')]
     assert actual_args.verbose == 3