Question mode keybindings++

This is an extension of the keybind branch.
I didn't want to push onto your branch, so I made this separate one.

This also implements the data structute I had in mind for the buffer, which is why commenting should now be trivial. Regardless of whether we're inside an answer or question, we can get the currently relevant data with `(get-text-property (point) 'sx-question-mode--data-here)`. 
Oh, whoops – good call including these in the filter.