table_formatter - allow user input for which classes to include

[hughdingb58]

Several of the current table formatter actions are for adding classes. Currently, various actions are hard coded to add specific classes, e.g. add the osfi-txt--bold attribute. It would be more flexible to have an action involving a textbox input where the user could themselves specify which classes they want to add.

This suggestion could potentially apply for removing tags (like the current action for removing p tags) as well.

Be sure to update the readme with any changes made here.