Support for array access through string.

I'm kind of ambivalent on this feature though: https://github.com/component/to-function/issues/4

Example from now README section, Equality:
```js
var users2 = ['Tobi', 'Loki', 'Tobi', 'Jane'];
console.log(users2.filter(_('Tobi'));
// => ['Tobi', 'Tobi']
```