Demo *.ink script

I suggested that it would be nice with a more elaborate demo ink script at some point in time. Sadly, the one's I'm normally writing contain a few too many jInk-specific constructs to be really useful as examples.

In the meantime, however, I took a moment to do the classic short [Cloak of Darkness](http://www.firthworks.com/roger/cloak/) as an ink script. It can be found at:
https://github.com/micabytes/jink/blob/master/sample/cloak_of_darkness.ink