language-specs: Add support for todo.txt format
The todo.txt format is a simple set of rules that make todo.txt both human and machine-readable. The format supports priorities, creation and completion dates, projects and contexts.
- Project website: http://todotxt.org
- Format definition: https://github.com/todotxt/todo.txt
Screenshots of gnome-text-editor (gtksourceview-5, Adwaita theme) and pluma (gtksourceview-4, Tango theme):
