allow to add a classname for inline text attributes

[rockwellll]

This was opened in #855 but seems abandoned since the migration from CoffeeScript to JavaScript.

Adds support for specifying a className option to custom text attributes.

Trix.config.textAttributes.background = {
  className: 'background-color',
  inheritable: false,
}

Multiple whitespace separated classes can be assigned as well,

Trix.config.textAttributes.background = {
  className: 'bg-red-500 text-white rounded-md', # class names courtesy of TailwindCSS
  inheritable: false,
}

[thiagomarinho]

hello :) are there plans to introduce this feature?