add support for todo.txt syntax

[tristan-k]

I recently changed horses and got into todo.txt. It seems to be more popular this days than taskpaper. I would love to see support for the syntax in taskpaperplus.

https://github.com/ginatrapani/todo.txt-cli/wiki/The-Todo.txt-Format

[richardneish]

The link above is empty, see https://github.com/todotxt/todo.txt for the format of todo.txt files.