add optional() method to Validator interface

express-validator/express-validator.d.ts

@@ -150,6 +150,10 @@ declare module ExpressValidator {
 		 * Will work against Visa, MasterCard, American Express, Discover, Diners Club, and JCB card numbering formats
 		 */
 		isCreditCard(): Validator;
+		/**
+		 * Check an input only when the input exists
+		 */
+		optional(): Validator;
 	}
 
 	interface Sanitizer {