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`.
Hmm, yeah I get what you're saying. Wonder how `request` manages to make that work though. I don't have time right now, but will try to look into how `request` does it, unless you beat me to it.