Make OptionsPlugin abstract

It was never intended to be instantiated, but rather extended.

Closes #208.

Author: ctrueden

src/main/java/org/scijava/options/OptionsPlugin.java

@@ -71,7 +71,9 @@
  * @author Barry DeZonia
  * @author Curtis Rueden
  */
-public class OptionsPlugin extends DynamicCommand implements SingletonPlugin {
+public abstract class OptionsPlugin extends DynamicCommand implements
+	SingletonPlugin
+{
 
 	// -- Parameters --