"Prompt before exiting" StorySetting

* Added an optional 'exitprompt' StorySetting which, when set, shows a
prompt when the player closes or reloads the story's page in the
browser. This uses window.onbeforeunload, and isn't guaranteed to be
supported in every browser.
* Wikifier.parse: input strings containing variable-like syntax no
longer cause the variable to be defaulted.