Add the ability to "hide" commands.

This does not yet change how commands are treated, but allows for this to be implemented in the future.

cloud-annotations/src/main/java/com/intellectualsites/commands/annotations/ProxiedBy.java

@@ -44,4 +44,11 @@ public @interface ProxiedBy {
      */
     @Nonnull String value();
 
+    /**
+     * Whether or not the proxying command should be {@link Hidden}
+     *
+     * @return {@code true} if the proxying command should be hidden, {@code false} if not
+     */
+    boolean hidden() default false;
+
 }