XKCD Content Provider support (take 2)

Heavily based off the existing Bash content provider, although no longer quite so much @khellang :)

![Looks like this](https://jabbrlive.blob.core.windows.net/jabbr-uploads/clipboard_f3b8.png)

Has alt text as the title of the image, so no spoilers.
You're passing 4 parameters, but your only using 3(?) of them in the actual `ContentFormat` string...is there are reason for this?