Allow custom queries in painless scripting language

docs/indexables.md

@@ -93,7 +93,13 @@ ElasticPress integrates with `WP_Query` if the `ep_integrate` parameter is passe
         'day'   => 1,
     ) );
     ```
+* ```custom_query``` (*string*)
 
+    ```custom_query``` allows the user to write a custom query in the painless scripting language [https://www.elastic.co/guide/en/elasticsearch/painless/current/index.html](https://www.elastic.co/guide/en/elasticsearch/painless/current/index.html).
+    ```php 
+          $args['custom_query'][] = "doc['meta.birthday'].value.getMonthValue() == 3";
+    ```      
+
 * ```date_query``` (*array*)
 
     ```date_query``` accepts an array of keys and values (array|string|int) to find posts created on

includes/classes/Indexable/Post/Post.php

@@ -1227,6 +1227,23 @@ function( $tax_query ) use ( $args ) {
 			$use_filters = true;
 		}
 
+		if ( ! empty( $args['custom_query'] ) ) {
+			foreach ( $args['custom_query'] as $custom_query ) {
+				$filter['bool']['must'][] = array(
+					'bool' => array(
+						'filter' => array(
+							'script' => array(
+								'script' => array(
+									'source' => $custom_query,
+									'lang'   => 'painless',
+								),
+							),
+						),
+					),
+				);
+			}
+		}
+
 		/**
 		 * Like WP_Query in search context, if no post_status is specified we default to "any". To
 		 * be safe you should ALWAYS specify the post_status parameter UNLIKE with WP_Query.