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`.
To be clear, in this case `got` should not call `req.end()` inside:

```js
stream.pipe(got(/*...*/))
```

But in this, it should:

```js
got(/*...*/);
```

Or user should always do this: `got(...).end()` which is not nice.