eval-label-file should be an optional argument

[delip]

Right now, omitting it causes a match error.

Exception in thread "main" scala.MatchError: None (of class scala.None$)
	at junto.Junto$.main(Junto.scala:29)
	at junto.Junto.main(Junto.scala)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at com.intellij.rt.execution.application.AppMain.main(AppMain.java:147)