manifest additions for javafx jar

Great plugin, it saved me a ton of work. 

One thing I needed that it did not do was to be able to add custom manifest entries to the jar created by the plugin.  I modified your plugin to include a mechanism to pass manifest entries down into the xml generated for the javafx ant task.  I used the same key that sbt uses to modify the manifest for the standard packaged jar just scoped to your task.  I figured it may be useful to someone else so your more than welcome to integrate the change into your plugin if you think it fits.

Thanks,
Spencer