Support xref aka cross reference link fix in adoc

[georgecao]

I use JBake version 2.6.4 and adoc to write some blog. JBake has two configuration properties to fix relative image src. And they are img.path.update and img.path.prepend.host.

In adoc, I add an image in the doc using image::images/Apache_Thrift_Architecture.png[Thrift], the two properties mentioned above and fix the image src to work fine, especially the blog content is add into the index page.

So in adoc, we also can add a reference to another doc by xref:a-gentle-introduction-to-multithreading.adoc[introduction] , like image , this will generate a <a href='''>...</> tag with relative path. This need fix too.

I am not sure if there are more links to fix in adoc or other type like markdown.

I also found #143 , but it's different from this. So this may need a fix.

[georgecao]

I already write some code to fix this like how img src is fixed. But let's make sure it's a problem.
@ancho @jonbullock

[jonbullock]

Linking #649