Added anchor to category sections (alshedivat#2006)

Added anchor to categories so one could navigate to the category section
from the URL:

e.g.,

https://alshedivat.github.io/al-folio/projects#fun

_pages/projects.md

@@ -14,7 +14,9 @@ horizontal: false
 {% if site.enable_project_categories and page.display_categories %}
   <!-- Display categorized projects -->
   {% for category in page.display_categories %}
-  <h2 class="category">{{ category }}</h2>
+  <a id="{{ category }}" href=".#{{ category }}">
+    <h2 class="category">{{ category }}</h2>
+  </a>
   {% assign categorized_projects = site.projects | where: "category", category %}
   {% assign sorted_projects = categorized_projects | sort: "importance" %}
   <!-- Generate cards for each project -->