Allow users to specify a custom class to add to generated HTML elements

[glutanimate]

It would be neat for users to be able to create custom buttons that both create an HTML element and equip it with a predefined CSS class. Coupled to Anki's templatting system, this would grant advanced users a very powerful tool to customize their card formatting.

Implementation specs:

• expand the add-on configuration with an additional action key that allows users to specify a CSS class for a button
• add a function that's capable of applying/removing that CSS class on selected text
• hook everything up, so that clicking on a button with a CSS class set does the following:
  • if formatting in question present: Remove formatting and CSS class
  • if formatting in question not present: Apply formatting and CSS class