rot13 option + example

* There's a rot13 option (*Per answer*. Makes it easier to compose the puzzle IMHO). It protects against *accidental* discovery of answers via "view source" etc. (people can always use showAnswers to cheat :smirk_cat:)
* The default example (NSA) is rot13, but it's easy to go switch to the original [non-rot13] example (look for "#CeilingKeith" comments in `index.html`)
* I've made relevant features you've added (showAnswers and reveal) rot13-compatible. Hope I didn't miss anything

Sorry for the weird commit history (I've changed from @jlweisbeck's fork to yours in mid-air). Hope it's not too messy.
Anyway, thanks for *your* code. Happy holidays :christmas_tree: