make min max average and sum snippets

snippets/average.md

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+### average
+
+Returns the average of an of two or more numbers/arrays.
+
+Use `Array.reduce()` to add each value to an accumulator, initialized with a value of `0`, divide by the `length` of the array.
+
+```js
+const average = (...arr) => [].concat(...arr).reduce((acc, val) => acc + val, 0) / arr.length;
+```
+
+```js
+average([1, 2, 3]); // 2
+```