Add countBy, update other By snippets

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

snippets/countBy.md

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+### countBy
+
+Groups the elements of an array based on the given function and returns the count of elements in each group.
+
+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 countBy = (arr, fn) =>
+  arr.map(typeof fn === 'function' ? fn : val => val[fn]).reduce((acc, val, i) => {
+    acc[val] = (acc[val] || 0) + 1;
+    return acc;
+  }, {});
+```
+
+```js
+countBy([6.1, 4.2, 6.3], Math.floor); // {4: 1, 6: 2}
+countBy(['one', 'two', 'three'], 'length'); // {3: 2, 5: 1}
+```