Add Google Analytics option

Fixes #3

exampleSite/config.toml

@@ -12,6 +12,8 @@ theme = "hugo-cactus-theme"
 	twitter = "Your Twitter account"
 	# Disable comments by leaving disqusShortname empty
 	disqusShortname = "spf13"
+	# Enter your tracking code to enable Google Analytics
+	googleAnalytics = ""
 	enableRSS = true
 
 	# The variables below are optionally too and can be used to

layouts/partials/js.html

@@ -1,4 +1,14 @@
 <script src="//ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
 <script src="{{ .Site.BaseURL }}js/main.js"></script>
 <script src="{{ .Site.BaseURL }}js/highlight.js"></script>
-<script>hljs.initHighlightingOnLoad();</script>
+<script>hljs.initHighlightingOnLoad();</script>
+
+{{ with .Site.Params.googleAnalytics }}
+{{ "<!-- Google Analytics -->" | safeHTML }}
+<script>
+  var _gaq=[['_setAccount','{{ . }}'],['_trackPageview']];
+  (function(d,t){var g=d.createElement(t),s=d.getElementsByTagName(t)[0];
+  g.src=('https:'==location.protocol?'//ssl':'//www')+'.google-analytics.com/ga.js';
+  s.parentNode.insertBefore(g,s)}(document,'script'));
+</script>
+{{ end }}