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++
Merge pull request #69 from jleechpe/sx-networks

Functionality for retrieving network information and keeping track of favorite questions.
Fix logic for updating cache and setting variables.

Turned (sx-network--ensure-user) into (sx-network--initialize) and added
as hook for initialization.
Correct naming of cache to correspond to sx-cache-get.
Replace (sx-favorites--ensure-favorite-list) with (sx-favorites--initialize).

Simplify initialization since it is run at startup rather than tested
for by functions.

Clean up (sx-favorites--update-site-favorites) to provide the same
values for new sites as for existing.