Replace unicode menu icon with inline SVG

- unicode menu icon doesn’t render pixel crisp (has some
anti-aliasing), whereas SVG renders pixel optimized
- using inline SVG (instead of a referenced, attached file, will save
an http request

lib/site_template/_includes/header.html

@@ -5,7 +5,17 @@
     <a class="site-title" href="/">{{ site.name }}</a>
 
     <nav class="site-nav">
-      <span class="menu-icon">&#9776;</span>
+      <span class="menu-icon">
+        <svg version="1.1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" x="0px" y="0px"
+           viewBox="0 0 18 15" enable-background="new 0 0 18 15" xml:space="preserve">
+          <path fill="#505050" d="M18,1.484c0,0.82-0.665,1.484-1.484,1.484H1.484C0.665,2.969,0,2.304,0,1.484l0,0C0,0.665,0.665,0,1.484,0
+            h15.031C17.335,0,18,0.665,18,1.484L18,1.484z"/>
+          <path fill="#505050" d="M18,7.516C18,8.335,17.335,9,16.516,9H1.484C0.665,9,0,8.335,0,7.516l0,0c0-0.82,0.665-1.484,1.484-1.484
+            h15.031C17.335,6.031,18,6.696,18,7.516L18,7.516z"/>
+          <path fill="#505050" d="M18,13.516C18,14.335,17.335,15,16.516,15H1.484C0.665,15,0,14.335,0,13.516l0,0
+            c0-0.82,0.665-1.484,1.484-1.484h15.031C17.335,12.031,18,12.696,18,13.516L18,13.516z"/>
+        </svg>
+      </span>
       <div class="trigger">
         <a class="page-link" href="/about">About</a>
         <a class="page-link" href="/projects">Projects</a>

lib/site_template/css/main.css

@@ -313,6 +313,8 @@ a:visited { color: #205caa; }
     line-height: 36px;
   }
 
+  .site-nav .menu-icon svg { width: 18px; height: 16px; }
+
   .site-nav .trigger {
     clear: both;
     margin-bottom: 5px;