Support sending ordered list of messages

[cdevienne]

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:

  sendMany : List (String) -> Cmd msg
  

• add a task-based API:

  sendTask : String -> Task Error ()
  

[process-bot]

Thanks for the issue! Make sure it satisfies this checklist. My human colleagues will appreciate it!

Here is what to expect next, and if anyone wants to comment, keep these things in mind.