Added allEqual + tests

snippets/allEqual.md

@@ -0,0 +1,14 @@
+### allEqual
+
+Check if all elements are equal
+
+Use `Array.every()` to check if all the elements of the array are the same as the first one.
+
+```js
+const allEqual = arr => arr.every(val => val === arr[0]);
+```
+
+```js
+allEqual([1, 2, 3, 4, 5, 6]); // false
+allEqual([1, 1, 1, 1]); // true
+```