support plugins custom step types

[amiturgman]

images, carrousel, etc..

[aribornstein]

I think this it might be more effecient to embed this in data instead of adding complexity to the step layer. Since images and carrousels and etc are just ways of abstracting text based ui

For example

  "id": "workout",
  "type": "prompt",
  "data": { 
    "type": "choice",
    "text": "What kind of workout do you do?",
    "image": "url(../img/sports.png)",
    "options":  [ "None", "Crossfit", "TRX", "Kung-Fu" ]
  }