Add countBy, update other By snippets

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

snippets/groupBy.md

@@ -6,8 +6,8 @@ Use `Array.map()` to map the values of an array to a function or property name.
 Use `Array.reduce()` to create an object, where the keys are produced from the mapped results.
 
 ```js
-const groupBy = (arr, func) =>
-  arr.map(typeof func === 'function' ? func : val => val[func]).reduce((acc, val, i) => {
+const groupBy = (arr, fn) =>
+  arr.map(typeof fn === 'function' ? fn : val => val[fn]).reduce((acc, val, i) => {
     acc[val] = (acc[val] || []).concat(arr[i]);
     return acc;
   }, {});