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/columns/ColDue.h

@@ -36,6 +36,7 @@ public:
   ColumnDue ();
   ~ColumnDue ();
 
+  bool validate (std::string&);
   void setStyle (const std::string&);
   void measure (Task&, int&, int&);
   void render (std::vector <std::string>&, Task&, int, Color&);