Support xref aka cross reference link fix in adoc

[georgecao]

I use JBake version 2.6.5 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. These two properties only affect img tag and it's src attribute.

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

I introduce three new properties, relative.path.update, relative.path.prepend.host and relative.tag.attribute. relative.path.update and relative.path.prepend.host are for configuring all relative paths updates, not just img tag. And the third property relative.tag.attribute defines which tags need fix, for example relative.tag.attribute=img=src,a=href.

For backward compatible, both the new and old properties should work.

With a false default value, we should use the properties this way
boolean pathUpdate = config.getImgPathUpdate() || config.getRelativePathUpdate()
With a true default value, we should use the properties this way:
boolean prependSiteHost = configuration.getImgPathPrependHost() && configuration.getRelativePathPrependHost();

[coveralls]

Coverage increased (+0.04%) to 80.827% when pulling c056e6a on georgecao:ru into c9989a3 on jbake-org:master.

[jonbullock]

Thanks for taking the time to raise this PR.

This area of functionality will be reviewed, hopefully for the 2.7.0 release, so it may be a while before this gets properly reviewed.

[georgecao]

Thanks for taking the time to raise this PR.

This area of functionality will be reviewed, hopefully for the 2.7.0 release, so it may be a while before this gets properly reviewed.

Good to know this.