Contextual placeholder arguments

[julianrubisch]

Feature

Description

Passes a record and index arguments to any futurize placeholder block rendered either for an ActiveRecord::Base or any type of collection.

I.e. you can do

<%= futurize Post.all .... do |post, index| %>
  <li><%= index + 1 %>. <%= post.title %></li>
<% end %>

Fixes #104

Checklist

• My code follows the style guidelines of this project
• Checks (StandardRB & Prettier-Standard) are passing

[julianrubisch]

@hopsoft could you take a cursory look at this PR and look if it does what you expect?