Enhancements - Cmd object

- New Cmd object to handle localized commands, customReports and general
  command parsing.
- Localized new Subst methods.
- Relocate guess method from parse.cpp to text.cpp.
- Converted Att object to use new valid/parse scheme.
- Unit tests for Cmd object.
- Fixed att.t.cpp unit tests.

src/task.h

@@ -43,6 +43,7 @@ bool validPriority (const std::string&);
 bool validDate (std::string&);
 bool validDuration (std::string&);
 bool validDescription (const std::string&);
+bool validCommand (std::string&);
 void loadCustomReports ();
 bool isCustomReport (const std::string&);
 void allCustomReports (std::vector <std::string>&);