Let the user define extra variants

[lcjury]

I'm in a situation where I need a variant for bigger icons, but I'm already providing sizes in my variant config files:

config.default_class = {
    solid: 'h-6 w-6',
    outline: 'h-6 w-6',
    mini: 'mr-1.5  h-5 w-5 flex-shrink-0 text-gray-400',
  }

The gem doesn't allow the user to define non-safe variants (solid, outline, mini).

I would happily provide a PR for this feature.