No markdown support

[xMTinkerer]

the messages post endpoint allows for a markdown option. It would be great to add this as a parameter to the sendMessage function.
It looks like it would just need to be a change this:

  send: (user, message, room) ->
    @robot.logger.debug "Sending message"
    @room_ids.forEach (room_id) =>
      @robot.logger.debug "send message to room #{room_id} with text #{message}"
      @spark.sendMessage
        roomId: room_id
        text: message

to something like

  send: (user, message, room, markdownText) ->
    @robot.logger.debug "Sending message"
    @room_ids.forEach (room_id) =>
      @robot.logger.debug "send message to room #{room_id} with text #{message}"
      @spark.sendMessage
        roomId: room_id
        text: message
        markdown: markdownText

Or would it make sense to pass the desired payload straight through? Something like:

  send: (user, payload, room) ->
    @robot.logger.debug "Sending message"
    @room_ids.forEach (room_id) =>
      @robot.logger.debug "send message to room #{room_id} with text #{message}"
      @spark.sendMessage payload

Where payload is:

var payload = {
  "text": "Some text here",
  "files": ["http://www.example.com/images/media.png"]
}

or

var payload = {
  "markdown": "Some `markdown` text **here**"
}

I'm not sure if that would mess with the generic adapter though?
Thoughts?

Edit: Formatting fixes