Proposed official virtual packages - todo and todo.txt

[Dave Steele <dsteele@gmail.com>]

Please update the Authoritative List of Virtual Package Names to
include "todo" and "todo.txt".

Discussion of the change is documented in [#976402].

[#976402]: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=976402


Suggested content:

# Miscellaneous

virtualPackages:
  - name: todo
    description: a command-line task management utility
  - name: todo.txt
    description: task management based on a standard free-form text format