Add cleanDiacritics method

types/underscore.string/index.d.ts

@@ -91,6 +91,13 @@ declare module 'underscore' {
          */
         clean(str: string): string;
 
+        /**
+         * Replace diacritic characters with closest ASCII equivalents.
+         * cleanDiacritics('ääkkönen') => 'aakkonen'
+         * @param str
+         */
+        cleanDiacritics(str: string): string;
+
         /**
          * Count occurences of a sub string.
          * ('Hello world', 'l') => 3