Add macros for JDK and Groovy JDK links.

This extends the current support for Groovy API macros to the JDK and the
Groovy JDK. It also allows the author to do some extra things:

* Add a custom label for a link using square brackets
* Link to the index page

As an example, you could link to the Groovy JDK index page with a dedicated
label using

    gjdk::index[Groovy JDK]

Even if you don't want to use a custom label, the square brackets can help
delimit class names at the end of a sentence:

    gapi::groovy.xml.MarkupBuilder[].

This should help immensely with cross-documentation links and I'm using it
already in the Groovy Development Kit chapter.