Add an "encoding" option for Groovydoc tool and Ant task

The Groovydoc tool and Ant task don't seem to support an "encoding" option. I guess this can be worked around by setting the "file.encoding" system property, but this is not always possible/desirable when running Groovydoc embedded in another app (e.g. Ant or Gradle). Therefore I suggest to add an explicit "encoding" option.