implement xkcd functionality

The strings.json file is extensible; other fields that can be
placed in the xkcd string include the {year}, {month} and {day}
of the comic; the {alt} text; and links to the {img}.
A {transcript} is also available.
update documentation
Merge pull request #253 from zuzak/xkcd-module

xkcd module
Don't pork up if UD is down