favicon support

[mikebeaton]

I was wondering, would there be any chance of including some favicon code in _includes/head.html?

Something like:

  {% if site.favicon != nil %}
    <link rel="shortcut icon" href="{{ site.favicon | absolute_url }}">
  {% endif %}

[pmarsceill]

This has been in my mental queue for a while now, feel free to open a PR if you'd like to contribute it to the next release.