Update syntax to use a subset of `Dash/Plus`

[unforswearing]

Dash/Plus was originally created by Patrick Rhone and includes potentially useful markup.

From the above link:

– (Dash): Undone Action Item. 
+ (Plus): Done Action Item. 
<- (Left Arrow): Delegated (with a note to whom and the date). 
-> (Right Arrow): Waiting – (i.e. for another action). 
^ (Triangle): Data Point. 
O (Circle): A circle around any of the above means that it has been carried forward, 
            moved to another list or otherwise changed status – i.e. an item has now 
            become an Action Item elsewhere (with a note about where that item has gone).

• ^ and > operators are already used in todo_markup.js and would need to be changed when adding Dash/Plus support
• Since Dash/Plus was intended for handwritten notes, a new operator for items that change status / moved / are delegated, to replace the circle O.

[unforswearing]

I don't think I will do this. Some thoughts after two weeks:

• The Dash as the undone item isn't necessary with !
• The Plus for completed items isn't necessary with x
• Operators for "delegated" and "waiting" may not be necessary with footnotes
• I like the idea of having an operator for data points, but when would I use this instead of a note or footnote?
• The moved / carried forward circle operator isn't necessary with notes and foot notes.

Dash/Plus may be fun to write as a standalone project. Will not add to todo_markup.