Support explainxkcd options

[quiddity-wp]

It might be useful to add an option to link to https://www.explainxkcd.com/ pages ?

E.g.
https://www.explainxkcd.com/wiki/index.php/1782
https://www.explainxkcd.com/wiki/index.php/1810
https://www.explainxkcd.com/wiki/index.php/1254

Just a thought, reject freely. :-)