Futago-za Ryuu

I actually like the idea of an `unexpected` rule, but I'm thinking of putting it in as optional via the `features` option. Is that fine with you @log4b0at?

Can start on this after stage 1 of #523, so should start thinking of what to include by default, and what should be included in the browser distribution as we...

@marchant Any update on this?

Updated the OP to reflect my current direction for this issue.