Support title-img config param to have image in the navbar instead of text

_includes/nav.html

@@ -7,7 +7,11 @@
         <span class="icon-bar"></span>
         <span class="icon-bar"></span>
       </button>
-      <a class="navbar-brand" href="{{ site.url }}">{{ site.title }}</a>
+      {% if site.title-img %}
+        <a class="navbar-brand navbar-brand-logo" href="{{ site.url }}"><img src="{{ site.title-img }}"/></a>
+      {% else %}
+        <a class="navbar-brand" href="{{ site.url }}">{{ site.title }}</a>
+      {% endif %}
     </div>
 
     <div class="collapse navbar-collapse" id="main-navbar">