Fix tj#256: automatically call addImplicitHelpCommand for sub-commands

Move the setting of executables to true from the command method’s handling
of its undocumented desc parameter to the description method.

index.js

@@ -177,7 +177,6 @@ Command.prototype.command = function(name, desc, opts) {
 
   if (desc) {
     cmd.description(desc);
-    this.executables = true;
     this._execs[cmd._name] = true;
     if (opts.isDefault) this.defaultExecutable = cmd._name;
   }
@@ -876,6 +875,7 @@ Command.prototype.version = function(str, flags) {
 
 Command.prototype.description = function(str, argsDescription) {
   if (arguments.length === 0) return this._description;
+  this.executables = true;
   this._description = str;
   this._argsDescription = argsDescription;
   return this;