Al folio main giscus pages (alshedivat#1596)

Solves alshedivat#1586 by adding the option for giscus comments sites using the
pages layout and minimally modifies Project 2 to show giscus comments as
an example

Author: DavidKrassnig

_layouts/page.html

@@ -21,4 +21,7 @@ <h2>References</h2>
           </div>
           {%- endif %}
 
+          {%- if site.giscus.repo and page.giscus_comments -%}
+            {% include giscus.html %}
+          {%- endif -%}
         </div>

_projects/2_project.md

@@ -1,10 +1,11 @@
 ---
 layout: page
 title: project 2
-description: a project with a background image
+description: a project with a background image and giscus comments
 img: assets/img/3.jpg
 importance: 2
 category: work
+giscus_comments: true
 ---
 
 Every project has a beautiful feature showcase page.