Keypad: provide Node 10 friendly "array flatten" mechanism :eyeroll:

Author: rwaldron

lib/keypad.js

@@ -10,14 +10,20 @@ const aliases = {
   hold: ["hold"]
 };
 
+function flatten(array) {
+  return array.flat ?
+    array.flat() :
+    array.reduce((accum, val) => accum.concat(val), []);
+}
+
 function flatKeys(options) {
   let keys = [];
 
   if (options.keys && Array.isArray(options.keys)) {
     keys = options.keys.slice();
 
     if (keys.every(Array.isArray)) {
-      keys = keys.flat();
+      keys = flatten(keys);
     }
   }