Add attr_value support to shortcuts

[Spone]

Related discussion: ViewComponent/view_component#677 (comment)

NOTE: This is a quick & dirty test! It should be refactored and tested before opening a PR against upstream.

This creates an attr_value param for shortcuts. You can pass it a Proc that will be called when the tag is parsed.

Slim::Parser.options[:shortcut].update({
  '~' => { attr: "class", attr_value: ->(v) { "styles['#{v}']" } }
})

Then in your slim template you can do:

~text Lorem ipsum
/ <div class="Styled_6c266_text">Lorem ipsum</div>

p~text.is-big LOREM IPSUM
/ <p class="Styled_6c266_text is-big">LOREM IPSUM</p>

[Spone]

Replaced by slim-template#903