Better organization of config/runtime keys

Currently, information about which config/runtime parameters wp-cli supports is scattered and duplicated in various places.

It would be nice to have a machine-readable format. For example:

```php
register_config_key( 'debug', array(
  'runtime' => true,
  'file' => true,
  'desc' => 'Suppress informational messages',
  'default' => false
) );
```

Having this, the [reference page](https://github.com/wp-cli/wp-cli/wiki/Configuration) could be generated automatically.

Recently: #267