Document how to extend turbo_stream for custom actions

[seanpdoyle]

Closes #554

The current Turbo::Streams::TagBuilder class supports the actions that Turbo provides out of the box.

For applications that define Custom Stream Actions, this new support aims to provide them with a mechanism to define Rails-side helpers.

For example, consider an [action="highlight"] custom stream. To build those with first-party tooling, applications could add a hook block to their initializer:

# config/initializers/turbo.rb
ActiveSupport.on_load :turbo_streams_tag_builder do
  def highlight(target)
    action :highlight, target
  end

  def highlight_all(targets)
    action_all :highlight, targets
  end
end

turbo_stream.highlight "my-element"
# => <turbo-stream action="highlight" target="my-element"><template></template></turbo-stream>

turbo_stream.highlight_all ".my-selector"
# => <turbo-stream action="highlight" targets=".my-selector"><template></template></turbo-stream>

[afcapel]

Thanks @seanpdoyle!