- 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.
- Added feature #800, adding a new command 'columns' that lists all the columns
available for custom reports, and includes their formatting options (thanks
to T. Charles Yun).
- 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.
