support them

aliases zones in the target zone configuration

This reverts commit b926d78c5c.

This commit adds support for zones aliases. This allows to define one or
multiple zone as aliases of an existing zone without using workarounds
like simlinks and miltiple "zones" entries in the configuration file.

An alias zone is share all of its content with it parent zone, only the
name of the zone is different.

```
zones:
  example.com.:
    aliases:
      - example.net.
      - example.org.
    sources:
      - in
    targets:
      - out
```

Known issues:
- No documentation,
- Only the `octodns-sync` and `octodns-validate` commands supports
aliases zones at this time,

I added a loop in the manager init function which convert all alias
zone to "real" ones during config validation, however I'm not sure
this is the right approach. Comments welcome.

That's all Dyn supports and it's cleaner to match it than to scale dyn since
we'd lose precision we can't get back during populate.

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
```