Implement body option

This option will allow to remove all code in `sent` and leave it only as API wrapper around `got`.

##### options.body

Type: `string`, `Buffer`, `ReadableStream`  

Body, that will be sent with `POST` request. If present in `options` and `options.method` is not set - `options.method` will be set to `POST`.
@sindresorhus I think we should leave it for next version thou. Seems like they restricting this feature to `POST` and `PUT` requests - if method is `PUT` and `POST` and callback is omitted - they will not call `req.end()`.