Add new variable pages that add one or more css files

[javierartero]

---
title: Welcome to marsman
body: home
active: home
css: home
---

refactor and improve the following code

    <% if (class_active = current_page.metadata[:page][:css].to_s) %>
      <% class_active.split(',').map(&:lstrip).each do |item| %>
        <%= stylesheet_pack_tag item %>
      <% end %>
    <% end %>