Added utilities for scripts

* setTransitionCSS() replaces the contents of the #transitionCSS
<script> element with the passed string. This allows you to completely
override the default transition CSS without it cascading down. I'm
offering this as a JS function because many transitions will also
require some extra JS, and being able to provide the complete transition
code in just a script is desirable.
* prerender and postrender are objects whose function properties are
executed every time the passage is rendered. The functions added to them
take one argument: the resultant .passage .content element. Their 'this'
value is the Passage object being rendered.
* Changed Jonah's .body element's class to .content, to match Sugarcane.
* Fixed a bug in Wikifier.prototype.fullArgs().