Expression support

- Added Context::getColumns to return a vector of column names.
  This is to help a shift toward using the Column objects to assist
  in the parsing/validation of data entry/modifications.
- Added Column::modifiable to delegate an attributes readable/writeable
  state.  This means the columns will be in charge of their own
  mutability, which will simplify and generalize Command::modify_task.

src/Context.h

@@ -58,6 +58,8 @@ public:
   int getWidth ();                     // determine terminal width
   int getHeight ();                    // determine terminal height
 
+  const std::vector <std::string> getColumns () const;
+
   bool color ();                       // TTY or <other>?
   bool verbose (const std::string&);   // Verbosity control