Attributes and tag transform handlers

I have added tag and attributes transform functionality.

It is a little bit similar to Pinpickle's solution but have a wider application (here is how he done this: https://github.com/Pinpickle/sanitize-html/commit/eb7245b87029b172a68eb1dd52b50d5765dd32c7).

I have included tests and usage for this functionality.

This solution is similar also to one provided by HTMLPurifier php library.