Use relative url function for custom CSS files (#132)

Signed-off-by: Daniele Ricci <daniele@casaricci.it>

layouts/_default/baseof.html

@@ -23,7 +23,7 @@
     {{ end }}
 
     {{ range .Site.Params.custom_css }}
-      <link rel="stylesheet" href="{{ $.Site.BaseURL }}{{ . }}">
+      <link rel="stylesheet" href="{{ relURL ($.Site.BaseURL) }}{{ . }}">
     {{ end }}
 
     {{ block "favicon" . }}