Create flatten-array-up-to-depth.md

In reference to #100.

snippets/flatten-array-up-to-depth.md

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+### Flatten array up to depth
+
+Use recursion, decrementing `depth` by 1 for each level of depth.
+Use `Array.reduce()` and `Array.concat()` to merge elements or arrays.
+Base case, for `depth` equal to `1` stops recursion.
+Omit the second element, `depth` to flatten only to a depth of `1` (single flatten).
+
+```js
+const flattenDepth = (arr, depth = 1) =>
+  depth != 1 ? arr.reduce((a, v) => a.concat(Array.isArray(v) ? flattenDepth (v, depth-1) : v), []) 
+  : arr.reduce((a,v) => a.concat(v),[]);
+// flattenDepth([1,[2],[[[3],4],5]], 2) -> [1,2,[3],4,5]
+```