Lcg/chef 12 attr

See https://gist.github.com/danielsdeleo/da9a8d9a91cc8c2781b0
Would be nice to include at least a little of the bad input here. Ideally, you could say `node[:foo][:bar] is [ "a", "b" ]` or something, but that requires you to keep another variable to track how deep you've gone, which seems like overkill. But we could say `"#{arg}" isn't a valid element reference for attribute value #{attr}"` or something like that.