Update Exercise 12 answer to ES6 format

[Writech]

[morenoh149]

I'd like to see global support above 80% before merging this in http://caniuse.com/#feat=arrow-functions thoughts?

[gabrielkunkel]

Is there a way to make it an option (two potential answers)? Arrow functions/lambdas still aren't intuitive for most of the community? It would be cool if answers could be toggled between ES5, ES6, and, even, TypeScript. :-)