- 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.
- Changed the ::measure and ::render methods from pure virtual to
virtual.
- Fixed bug where recur.indicator used the length of the indicator
instead of the indicator.
- Implemented ColString.{h,cpp} to support generic Views based on
strings, not tasks.
- Implemented newly virtual Column:: methods.
