Support meta_query argument in query integration

This PR adds support for `meta_query`. It supports queries like so:

 ```php
    new WP_Query( array(
        's'          => 'search phrase',
        'meta_query' => array(
            array(
                'key'   => 'key_name',
                'value' => 'meta value',
                'compare' => '=',
            ),
            array(
                'key'   => 'key_name2',
                'value' => 'meta value',
                'compare' => '!=',
            ),
            'relation' => 'AND',
        ),
    ) );
```

With this PR `compare` supports `!=`, `=`, `EXISTS`, and `NOT EXISTS`. `relation` supports `OR` and `AND.