Add AdhocCommandHandlers::add_command to simplify the usage of this class

And make things a little bit clearer

louloulibs/xmpp/adhoc_commands_handler.cpp

@@ -15,9 +15,9 @@ const std::map<const std::string, const AdhocCommand>& AdhocCommandsHandler::get
   return this->commands;
 }
 
-std::map<const std::string, const AdhocCommand>& AdhocCommandsHandler::get_commands()
+void AdhocCommandsHandler::add_command(std::string name, AdhocCommand command)
 {
-  return this->commands;
+  this->commands.emplace(std::make_pair(std::move(name), std::move(command)));
 }
 
 XmlNode AdhocCommandsHandler::handle_request(const std::string& executor_jid, const std::string& to, XmlNode command_node)

louloulibs/xmpp/adhoc_commands_handler.hpp

@@ -31,9 +31,9 @@ public:
    */
   const std::map<const std::string, const AdhocCommand>& get_commands() const;
   /**
-   * This one can be used to add new commands.
+   * Add a command into the list, associated with the given name
    */
-  std::map<const std::string, const AdhocCommand>& get_commands();
+  void add_command(std::string name, AdhocCommand command);
   /**
    * Find the requested command, create a new session or use an existing
    * one, and process the request (provide a new form, an error, or a