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/custom.html

@@ -100,6 +100,7 @@ report.mine.sort=priority-,project+</pre></code>
                   <li>active
                   <li>tags
                   <li>recur
+                  <li>description_only
                   <li>description
                 </ul>