Add-on

- Updated the import-todo.sh with the latest examples from todotxt.org, and
  tested that the import is still good.
A3t

- Combined ::findFileOverride and ::findConfigOverride into ::findOverride.
- Made ::findBinary public, and corrected comment.
Command

- Added a debug mode parse tree dump in Command::filter, which is the
  first opportunity to see the complete tree.  There will be other places
  where this occurs too.
CmdCustom

- Custom report filters are now tagged acordingly in the parse tree.
A3t

- Default and assumed commands are tagged accordingly.
A3t

- Implemented ::findFilter to identify nodes that comprise a filter.
A3t

- Implemented ::findModifications to identify nodes that comprise the
  task modifications.
A3t

- Cleaned up ::validate, added notes for further validation.
- Removed obsolete comments.
A3t

- ::findFilter and ::findModications now remove the '?' tag.