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`.
> But in this, it should:

I'm uncertain what you mean now.

`got(/*...*/);` isn't actually valid by itself. Do you mean `got(/*...*/, function () {});`?