feat: support for custom head content (#59)

_includes/custom-head.html

@@ -0,0 +1,6 @@
+{% comment %}
+  Placeholder to allow defining custom head, in principle, you can add anything here, e.g. favicons:
+
+  1. Head over to https://realfavicongenerator.net/ to add your own favicons.
+  2. Customize default _includes/custom-head.html in your source directory and insert the given code snippet.
+{% endcomment %}

_includes/head.html

@@ -14,4 +14,5 @@
   {%- include extensions/google-analytics.html -%}
   {%- endif -%}
   {%- include extensions/code-highlight.html -%}
+  {%- include custom-head.html -%}
 </head>