Fixed commands without implementation

This makes sens to declare help on "intermediate" commands such as
"show".

Problem was that commands would have an implementation method registered
even if no implementation was specified.

Author: promovicz

command-interface/macros.dylan

@@ -33,7 +33,7 @@ define macro command-aux-definer
                             ?bindings;
                             ?implementation;
                           end method;
-           let %command = build-command(%root, %symbols, handler: %handler, ?keywords);
+           let %command = build-command(%root, %symbols, ?keywords);
            ?parameters
          end }
 
@@ -71,6 +71,8 @@ define macro command-aux-definer
     { } => { }
     { help ?text:expression; ... }
       => { help: ?text, ... }
+    { implementation ?:expression; ... }
+      => { handler: %handler, ... }
     { ?other:*; ... } => { ... }
 
   // definitions that define parameters