Enhancement - annotations

- Added support for "annotate" command to annotate existing tasks.
- Bumped file format to version 3, due to the annotations.
- Added unit tests to verify that annotations work.
- Changed 'description' column everywhere to include annotations.
- Added 'description_only' column to exclude the annotations.
- Fixed bug in Table.cpp that calculated the width of multi-line
  columns by using the cell length, instead of the length of the
  longest individual line.
- Updated documentation with new feature.
- Updated documentation with new column.
- Enhanced t.t unit tests to cover format 43

html/task.html

@@ -108,6 +108,10 @@
                   <li>Added support for the "echo.command" configuration variable that displays
                       the task affected by the start, stop, do, undo, delete and undelete
                       commands (thanks to Bruce Dillahunty).
+                  <li>Added support for task annotations, with each annotation comprising a
+                      timestamp and a description.
+                  <li>Added support for a 'description_only' column that can be used in custom
+                      reports which excludes annotations.
                 </ul>
 
                 <p>