create a facility to limit the class scope of the plugin

[hgschmie]

Limit the set of classes on which the plugin acts.

[mbellomo]

You asked for some use cases, so here's mine.

I only care about duplicates that our company's code creates.
I only care about resources, NOT classes
There's a lot of duplicate classes in java libraries, especially the XML libraries. Example:
[WARNING] Found duplicate classes in [stax:stax-api:1.0.1,xml-apis:xml-apis:1.3.04] :
[WARNING] javax.xml.XMLConstants
[WARNING] javax.xml.namespace.NamespaceContext
[WARNING] javax.xml.namespace.QName

Relevant parts of dependency tree:
[INFO] +- org.apache.xmlbeans:xmlbeans:jar:2.6.0:compile
[INFO] | - stax:stax-api:jar:1.0.1:compile
[INFO] +- xalan:xalan:jar:2.7.1:compile
[INFO] | - xalan:serializer:jar:2.7.1:compile
[INFO] | - xml-apis:xml-apis:jar:1.3.04:compile

Now I realize that I have no idea what version of that class is getting loaded each time, and that is a bad thing. But older (pre-maven/ivy/dependency-management) libraries often didn't care and copied classes around to ensure they were there for you. But for my use case, I just want to make sure that things aren't colliding.

[mbellomo]

I finally got around to upgrading to the basepom version of this plugin in place of the older ning version. And this issue immediately cropped up again. I've updated my patch to the work with the most current version. Will submit a pull request.