Allow adding classpath for custom rules written in java.

[martenbohlin]

If you create your own PMD rules in java we need a way to set the classpath where the plugin can load those classes and resources from.

[martenbohlin]

Hi @amitdev ! Anything I can do to get this PR accepted? It would help our organisation leverage PMD to enforce code rules we think applies to us but would probably not be of use to others since they are to specific of how we want the code to look in our code base.

[jborgers]

Not sure why this was not merged earlier. Maybe because it introduces many changes and difficult to oversee and maybe difficult to test. Currently it has conflicts which need to be solved.

[martenbohlin]

Conflicts has been resolved, and I have tested that it still works.

For us it would be great to get support for custom rules written in java.