code: function validateCards(cards) {
    if (!Array.isArray(cards)) {
      throw new Error('`cards` must be passed as an array');
    }
    for (let i=0; i < cards.length; i++) {
      let card = cards[i];
      if (card.type !== RENDER_TYPE) {
        throw new Error(`Card "${card.name}" must be type "${RENDER_TYPE}", was "${card.type}"`);
      }
      if (!card.render) {
        throw new Error(`Card "${card.name}" must define \`render\``);
      }
    }
  }
docs: runtime Text renderer
  renders a mobiledoc to Text
  
  input: mobiledoc
  output: Text (string)