Deep flatten and housekeeping

Changed original flatten to be named deepFlatten, added normal flatten, improved some other snippets.

snippets/deep-flatten-array.md

@@ -0,0 +1,10 @@
+### Deep flatten array
+
+Use recursion.
+Use `reduce()` to get all elements that are not arrays, flatten each element that is an array.
+
+```js
+const deepFlatten = arr =>
+  arr.reduce( (a, v) => a.concat( Array.isArray(v) ? flatten(v) : v ), []);
+// deepFlatten([1,[2],[[3],4],5]) -> [1,2,3,4,5]
+```