Add API calls and stubs for filter matching and subscription management

shell/src/Command.h

 #ifndef COMMAND_H
 #define COMMAND_H
 
 #include <map>
 #include <stdexcept>
 #include <string>
 
 class Command
 {
 public:
   const std::string name;
 
   explicit Command(const std::string& name);
   virtual ~Command();
   virtual void operator()(const std::string& arguments) = 0;
   virtual std::string GetDescription() const = 0;
   virtual std::string GetUsage() const = 0;
+
+protected:
+  void ShowUsage() const;
 };
 
 typedef std::map<const std::string, Command*> CommandMap;
 
 class NoSuchCommandError : public std::runtime_error
 {
 public:
   explicit NoSuchCommandError(const std::string& commandName);
 };
 
 #endif