Set text property sx-question-mode--data-here

It holds the data of current section, which is a question or answer for
now. This means the data of thing-at-point is always immediately
available through get-text-property.
Define -visit to use answer link if available.
Merge branch 'master' into question-mode-keybindings++
Merge pull request #67 from vermiculus/question-mode-keybindings++

Question mode keybindings++