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.cpp

@@ -411,6 +411,17 @@ void Context::shadow ()
 */
 }
 
+////////////////////////////////////////////////////////////////////////////////
+const std::vector <std::string> Context::getColumns () const
+{
+  std::vector <std::string> output;
+  std::map <std::string, Column*>::const_iterator i;
+  for (i = columns.begin (); i != columns.end (); ++i)
+    output.push_back (i->first);
+
+  return output;
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 void Context::assumeLocations ()
 {