Add Clojure to highlight.js, add code highlight example to README.

By default reveal.js is configured with highlight.js for code syntax highlighting, but not for Clojure. This pull requests updates highlight.js with Clojure support so that a section like this:

```html
<section>
	<p align="left">
		<code class="clojure">
			<pre>
(def lazy-fib
  (concat
   [0 1]
   ((fn rfib [a b]
        (lazy-cons (+ a b) (rfib b (+ a b)))) 0 1)))
			</pre>
		</code>
	</p>
</section>

```

Will render like this:

```clojure
(def lazy-fib
  (concat
   [0 1]
   ((fn rfib [a b]
        (lazy-cons (+ a b) (rfib b (+ a b)))) 0 1)))
```

This pull request also bumps the README with a section on code syntax highlighting.

cc: @danhammer