Commands

- New 'exec' command that runs external programs, implemented by
  CmdExec.
- Restructured code so that only Commands implements ::read_only
  and ::displays_id methods, while derived objects simply set flags.
- Corrected column alignment in summary report.

src/commands/CmdExec.h

@@ -34,12 +34,12 @@ class CmdExec : public Command
 {
 public:
   CmdExec ();
-  ~CmdExec ();
 
-  bool implements (const std::string&) const;
+  bool implements (const std::string&);
   int execute (const std::string&, std::string&);
 
 private:
+  std::string _external_command;
 };
 
 #endif