Get interdocument xrefs working about docgist HOT 2 CLOSED

@mojavelinux Once I learn how to hook into asciidoctor.js to do this, the same logic can be injected into the document processing instead of being applied afterwards.

The xrefs on the example page are now actual xrefs: http://gist.asciidoctor.org/
Under the hood the source is a github file, which will be obvious when following the interdoc xrefs.
Public Dropbox folder: http://gist.asciidoctor.org/?dropbox-10666617%2FAsciiDoc%2Fdocument-a.adoc
Riseup pad: http://gist.asciidoctor.org/?riseup-docgist
Piratenpad: http://gist.asciidoctor.org/?epsp-piratenpad.de-docgist

from docgist.

Nice work.

Unfortunately, this isn't really an extension point, so the only way I can think of doing it atm is using a postprocessor step, as you have done.

from docgist.