$allowedtags as a string is lame

Without diving into core code, I'm assuming that you are allowing `$allowedtags` as a comma delimited string because this was a cool thing WP once did. I think an array should be forced here. A comma delimited list just doesn't allow for the expressiveness of a multidimensional array to indicate allowed elements and attributes.

Even if this is to allow core compat, I would argue that since this is a new function/API, we don't have to support the older method of handling `$allowedtags`.