Implement opts.content option

This option will allow to remove all code in `sent` and leave it only as API wrapper around `got`.

##### options.content

Type: `string`, `Buffer`, `ReadableStream`  

Content, 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 this is neat feature, but implementation is not quite clear to me.

Since all HTTP requests can have body, should we wait for content in this case?:

```js
got('http://somewhere.com');
```