Deep flatten and housekeeping

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

snippets/count-occurrences-of-a-value-in-array.md

@@ -3,6 +3,6 @@
 Use `reduce()` to increment a counter each time you encounter the specific value inside the array.
 
 ```js
-const countOccurrences = (arr, value) => arr.reduce((a, v) => v===value ? a + 1 : a + 0, 0);
+const countOccurrences = (arr, value) => arr.reduce((a, v) => v === value ? a + 1 : a + 0, 0);
 // countOccurrences([1,1,2,1,2,3], 1) -> 3
 ```