update renderer to render cards

[Cory Forsyth]

Update the dom renderer to render cards as described in bustle/mobiledoc-kit#235. Checklist copied from there:

• Change card shape to object with type, name, render and optional edit properties
• Card's type is validated by renderer (dom renderer cannot render 'text', e.g.)
• Change arguments passed by editor to card's render (or edit) method
  • single argument object with env, options and payload properties
• Return value of card's render (or edit) method is appended/concatenated by renderer
• card can register teardown callback by calling onTeardown(teardownFn)
• Change editor-dom renderer to clear child elements from card element on teardown
• Renderer constructor signature changes to accept options object with: cards, atoms, cardOptions, unknownCardHandler, editor (for editor-dom)
• Renderer render instance method accepts mobiledoc argument, returns rendered object with 2 props:
  • result property (which is a dom node or string, depending)
  • teardown method with no args tears down rendered mobiledoc (removing dom when applicable, calling registered card teardown callbacks when applicable)
• unknownCardHandler method is called whenever a renderer encounters an unknown card
• remove rendered sections from dom on teardown