Decide what to do about "\u180E" / ᠎

From https://github.com/es-shims/es5-shim/pull/194#issuecomment-31375159:

Here's a proposal by google to change that character from the "Zs" to "Cf" which might explain why they treat it as such: http://www.unicode.org/L2/L2013/13004-vowel-sep-change.pdf

Let's keep an eye on that proposal and follow up with TC39 if that character needs to be removed from the spec in ES6 (it's unlikely the ES5 spec will change)