Support sending ordered list of messages

In some case we have to send several messages in the socket at once, but in a specific order, which cannot be guaranteed by using several Cmd.

I can see 2 possible APIs for that:
- add a specific function:
  ```elm
  sendMany : List (String) -> Cmd msg
  ```

- add a task-based API:
  ```elm
  sendTask : String -> Task Error ()
  ```