Signed-off-by: Christian Funkhouser <cfunkhouser@heroku.com>

The SplitYamlProvider itself now requires a directory matching the
zone name under its directory to contain all YAML files. This doesn't
actually change the intended usage at all, just how the configuration
file is laid out.

Signed-off-by: Christian Funkhouser <cfunkhouser@heroku.com>

Signed-off-by: Christian Funkhouser <cfunkhouser@heroku.com>

Refactors the provider class lookup and kwarg processing so that it can be
reused for plan_output.

`Included` and `Excluded` can be used to filter records for one or more specific provider(s). This can be extremely useful when certain record types are not supported by a provider and you want only that provider to receive an alternative record.

See also: https://github.com/github/octodns/issues/26

```yaml
ignored:
  octodns:
    ignored: true
  type: A
  value: 1.2.3.4
```