Add optional "busy" slot to show activity

[hopsoft]

Adds an optional busy slot to toggle triggers and targets to show activity whenever a command roundtrips to the server.

Resolves #24

Depends on hopsoft/turbo_boost-streams#41

This PR adds the "busy" content feature. Busy content can be used to indicate activity to the user.

<%= tag.div id: dom_id(record) do %>
  <%= toggle_trigger_tag renders: current_partial_path, morphs: dom_id(record), 
    controls: dom_id(record, :example), locals: local_assigns do %>
    <%= toggle_busy_tag do %>
    Loading.... (only shown when busy)
    <% end %>
    <a>Toggle the Target</a>
  <% end %>

  <%= toggle_target_tag dom_id(record, :example) do %>
    This block is only executed if the target is "expanded."

    Toggled content...    
    <%= render "path/to/toggled/content" %>

    <%= toggle_busy_tag do %>
      Loading.... (only shown when busy if the target is expanded)
    <% end %>
  <% end %>
<% end %>

The following UX "best practices" are implicitly applied to the busy content.

• Busy content is only shown if the remote toggle command takes longer than 100 milliseconds to return
• Busy content remains visible for a minimum of 400 milliseconds

NOTE: Both the trigger and target can define busy content.