[#10] Implemented missing features form the RFC

- Modified the `init()` callback function so that it receives the value of `last-event-id`.
- `init()` can now return `{no_content, NewReq}` which generates a response with status `204 No Content`.
- All fields in an `event()` that are not one of `id`, `name`, `data` or `retry` are considered a comment.