Parse booleans & numbers

``` js
// now
Qs.parse('a=true');    // { a: 'true' }
Qs.parse('a=21.31');    // { a: '21.31' }
// but I would expect boolean
var obj = Qs.parse('a=true');    // { a: true }
Qs.parse('a=21.31');    // { a: 21.31 }
```

Currently I use `express-query-boolean` package which parses `true` and `false` strings into boolean but it would be great to have this feature included in `qs`.
