add support for tags

lib/html_head.typ

@@ -17,6 +17,8 @@
   description: none,
   /// Author of the site for metadata, default to document.author
   author: none,
+  /// Tags describing the content of the page
+  tags: (),
   /// Additional stylesheet for the page: must be a list of url
   stylesheets: (),
   /// List of related sites for metadata
@@ -38,6 +40,11 @@
     // Only set the Referer header for out request to the same origin
     html.meta(name: "referrer", content: "same-origin")
 
+    // Tags
+    if tags.len() != 0 {
+      html.meta(name: "keywords", content: tags.join(", "))
+    }
+
     if icon != none {
       html.link(rel: "icon", type: "image/png", href: icon)
     }