Support/Document way to extend Logstash's classpath

Certain plugins require extending the classpath (kafka input/output for custom serializers), and we leave it up to the end user to figure out how to approach it.

For example, _today_ you can add your library files to the `CLASSPATH` environment variable, or drop them in `vendor/jruby/lib`

It'd be great to have a tried n' true (and documented) way to do this. Maybe an option in the `jvm.options` file?