Make command more extendable (Commander interface).

@@ -59,9 +59,9 @@ public class Command {
         return state(state.getCommandValue());
     }
 
-    private Command command(Fields command, String argument)
+    protected Command command(Commander commander, String argument)
     {
-        this.command.append(" ").append(command.getCommand()).append(" ").append(argument);
+        this.command.append(" ").append(commander.getCommand()).append(" ").append(argument);
         return this;
     }
 }

+package pl.com.it_crowd.youtrack.api;
+
+public interface Commander {
+// -------------------------- OTHER METHODS --------------------------
+
+    String getCommand();
+}

 package pl.com.it_crowd.youtrack.api.defaults;
 
-public enum Fields {
+import pl.com.it_crowd.youtrack.api.Commander;
+
+public enum Fields implements Commander {
     assignee,
     issueId("issue id"),
     state,