aliases zones in the target zone configuration

This reverts commit b926d78c5c.

It can be useful to only synchronize zones that use a certain source. For
example, in a situation where some zones use a dynamic source and others don't,
you probably want to synchronize those with a dynamic source regularly, and
only synchronize the others when a change is made.

Although we only synchronize the zones that use a given source, we still want
to synchronize all sources to avoid deleting records that would live in another
source of the zone.

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.

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>

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

Support replace=True in zone.add_record

Thanks @offmindby!