Ace Editor has feature to disable printMargin

Argument `margin` is now supported by field `ace_editor` to set print margin visibility. Accepts boolean(bool).

```php
$fields = array(
    'id'       => 'css_editor',
    'type'     => 'ace_editor',
    'title'    => __('CSS Code', 'redux-framework-demo'),
    'subtitle' => __('Paste your CSS code here.', 'redux-framework-demo'),
    'margin'   => true, // (New Feature introduced) By default it is set to false, to enable simply set true
    'mode'     => 'css',
    'theme'    => 'monokai',
    'desc'     => 'Some description goes here....hehe',
    'default'  => "#header{\nmargin: 0 auto;\n}"
);
```

This actually sets *(bool)* to `editor.setShowPrintMargin(true/false);` and fixes #1663 with :heart: !