Prettify:  initial port of v3.x plugin

* Only initializes and runs plugin once, regardless of number of
  `<pre class="prettyprint">` elements on page.
* Exposes `window.prettyPrint()` method to other plugins via
  `prettyprint.wb-prettify` event.
Merge pull request #3134 from patheard/prettify

Prettify:  initial port of v3.x plugin