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`.
Instead of supporting a ReadableStream here, should we rather have `got` also be a writable stream, like `request`.