Adds `_.rchop` (with README, tests)

As I've added to the README:

**rchop** _.rchop(string, step)

```javascript
_.rchop('whitespace', 3)
=> ['w', 'hit', 'esp', 'ace']
```

Mirrors `_.chop` (and adds `lchop` as an alias for `chop`). Comes with tests.