Support for categories

I'd really like to see junit-interface add support for [junit categories](https://github.com/junit-team/junit/wiki/Categories). This is something that [both Maven and Gradle support](https://github.com/junit-team/junit/wiki/Categories), but SBT does not, making testing in Java difficult in Play framework.

I was thinking it'd make sense to add --includeCategories and --excludeCategories as options both taking a comma separated list of fully qualified class names.

Note that junit category annotations may reside on either the class or method.

I also found this being [asked for on the mailing list](https://groups.google.com/forum/#!topic/simple-build-tool/cNK5in78kpI).
i might replace 21 with "--include-categories=".length() so that it's clearer what the 21 is