Add countBy, update other By snippets

Updated all By snippets to have matching behavior, consistent argument naming and added countBy as per #100.

snippets/sumBy.md

@@ -5,9 +5,11 @@ Returns the sum of an array, after mapping each element to a value using the pro
 Use `Array.map()` to map each element to the value returned by `fn`, `Array.reduce()` to add each value to an accumulator, initialized with a value of `0`.
 
 ```js
-const sumBy = (arr, fn) => arr.map(fn).reduce((acc, val) => acc + val, 0);
+const sumBy = (arr, fn) =>
+  arr.map(typeof fn === 'function' ? fn : val => val[fn]).reduce((acc, val) => acc + val, 0);
 ```
 
 ```js
 sumBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], o => o.n); // 20
+sumBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], 'n'); // 20
 ```