Bring the todo formats back to life

[marph91]

They were implemented by using Joplin's "Todo Notes". This is not anymore available in plain Markdown. Probably the obvious option is to implement checklists in a single Markdown file.

The following formats are affected:

• Todo.txt
• Todoist
• Toodledo
• [x]it

The old documentation and implementation is still available for reference.