Re: [dom] DOMTokenList hook to validate added token is in a supported set of keywords (#103)

I've made the changes so that `add()` and `toggle()` will not add invalid tokens. I also added case insensitivity to the validation step. I did not add a validation step to the setter, per discussion on [irc](http://krijnhoetmer.nl/irc-logs/whatwg/20151104#l-272).

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