If someone wants to run without a whitelist, hook `__return_true`.

Review feedback: extend filters to allow additions (and only additions) to blacklists and whitelists set in constants

Adopts `\WP_CLI\Configurator\extract_assoc()`

From the PHP manual:

```
In PHP 5, value must be a scalar value.... In PHP 7, array values are also accepted.
```

Don't bother retrying. Will update readme to explain the errors the plugin provides, and emphasize the need for a CLI runner.

```
$ phpenv global 5.3
rbenv: version `5.3' not installed
```