Minileven: add filter to control where Featured Images are displayed

Allow plugins to change where Featured Images are displayed

That would allow a plugin to change the default behavior with a simple function like so:

```php
function jeherve_custom_minileven_thumb( $enabled ) {
        // Only display the Featured Images on single posts
        $enabled = ( '1' == get_option( 'wp_mobile_featured_images' ) ) && ( is_single() );

        return $enabled;
}
add_filter( 'minileven_post_thumbnails_supported', 'jeherve_custom_minileven_thumb' );
```