Added extension annotations describing compatibility across spec versions

I would appreciate some feedback on the verbiage and general display of this information. With the XSLT defined in this pull request, this example XML snippet:

```
   <depends>
    <api version="1.0"/> <!-- This is an existing tag -->
    <core version="2.0">
      <glsl version="300 es"/> <!-- optional -->
      <addendum>SOME_ENUM support was dropped</addendum> <!-- optional -->
    </core>
  </depends>
```

Would produce the following output:

> ### Dependencies
> Written against the [WebGL API 1.0](http://www.khronos.org/registry/webgl/specs/1.0/) specification.
> Promoted to core in [WebGL API 2.0](http://www.khronos.org/registry/webgl/specs/2.0/) specification. Requires GLSL #version 300 es.
> The following behavioral changes apply in the core spec:
> * SOME_ENUM support was dropped
"requires" is a new processing mode introduced on line 202.