Make props for Gren optional

Author: alexcanessa

lib/src/Gren.js

@@ -22,7 +22,7 @@ const defaults = {
 
 /** Class creating release notes and changelog notes */
 class Gren {
-    constructor(props) {
+    constructor(props = {}) {
         this.options = ObjectAssign({}, defaults, props);
         this.tasks = [];