Columns

- Refactored column objects to contain a ::validate method, for the
  validation of incoming data.
- Context.columns is now a vector of one of each column object,
  indexed by attribute name, for validation purposes.

src/Context.h

@@ -106,6 +106,7 @@ public:
 */
 
   std::map <std::string, Command*>    commands;
+  std::map <std::string, Column*>     columns;
 
   int                                 terminal_width;
   int                                 terminal_height;