Support for todo.txt

[Ypot]

Basic syntax:
https://github.com/todotxt/todo.txt/blob/master/README.md

Optional everything related with dates other than "due:date" , since Gina, the creator, believes it is better an integration with an extenal calendar, than to make the text more difficult to read, and to parse:
Treshold dates
t:
Recurring tasks:
rec:
https://github.com/paulroub/todo.txt-recurring-tasks/blob/master/README.md

Hidden
h:1

Nested projects (hierarchy) :
Far from being official, but it would be neat with orgzly's indentation.
Example:
+bigwork\stage1
todotxt/todo.txt#13