introduce a hightlightJsVersion variable in the config to control highli...

Had some issue with Java syntax highligths when using generics, see https://hk2.java.net/2.2.0-b24/custom-resolver-example.html.
Figured that version 7.5 fixes the issue.

However only this is hardcoded currently and only one version of the reflow maven skin is published.
This change is about exposing highlightjs's version to configuration in site.xml.