Deep support

It would be awesome if we could add a `deep` option that decamlize keys recursively inside objects and arrays.

e.g.

``` javascript
const input = { aA: [{ bB: 1 }, { cC: 2 }] }
const output = decamelizeKeys(input, '-', { deep: true })

// { 'a-a': [{ 'b-b': 1 }, { 'c-c': 2 }]}
```
