Add XKCD

I don't understand how could I not have it there for so long.

dotfiles/bmo/uris.ini

     tor     = https://theoldreader.com/
     tw      = https://twitter.com/
     w       = http://en.wikipedia.org/
+    xkcd    = http://xkcd.com/
     zoo     = http://www.zooborns.com/
 
 [uris.query]