Added custom elements to better format slide decks

Added the following:
 * chapter
 * intro
 * chapter-title
 * intro-title

This allows one to now simply do the following:
```clojure
(chapter :title "Some Title"
  (slide :title "1st chapter, 1st slide"
          ...))
```
Note that:
 * intro-title sets title to `<h1>`,
 * chapter-title sets it to `<h2>`, 
 * slide-title sets it to `<h3>`