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

src/task.h

@@ -88,6 +88,7 @@ std::string handleStart (TDB&, T&, Config&);
 std::string handleStop (TDB&, T&, Config&);
 std::string handleUndo (TDB&, T&, Config&);
 std::string handleColor (Config&);
+std::string handleAnnotate (TDB&, T&, Config&);
 
 // report.cpp
 void filter (std::vector<T>&, T&);