Allow sending class name as part of the attributes of the selected element

[diegoquinteiro]

We are excluding the class attribute of selected DOM elements. Some of the rules e.g. H1Rule expects the class attribute to be used to set properties.

To solve this issue it is NOT enough with removing class from the list of excluded attributes since as part of the highlighting process we add new classes to the element. We will have to remove all the classes added by us first.