Add a new cachify_prefetch helper

This generates <link rel="prefetch"> tags which provide prefetching
hints to browsers that support this HTML5 feature.

lib/connect-cachify.js

@@ -51,12 +51,14 @@ exports.setup = function (assets, options) {
         true_path, exists;
 
     if (typeof resp.local === 'function') {
+      resp.local('cachify_prefetch', cachify_prefetch);
       resp.local('cachify_js', cachify_js);
       resp.local('cachify_css', cachify_css);
       resp.local('cachify', cachify);
     }
     else if (typeof resp.locals === 'function') {
       resp.locals({
+        cachify_prefetch: cachify_prefetch,
         cachify_js: cachify_js,
         cachify_css: cachify_css,
         cachify: cachify
@@ -187,6 +189,13 @@ var cachify_css = exports.cachify_css = function (filename, options) {
                           options.hash);
 };
 
+var cachify_prefetch = exports.cachify_prefetch = function (filename, options) {
+  if (! options) options = {};
+  return prod_or_dev_tags(filename,
+                          '<link rel="prefetch" href="%s">',
+                          options.hash);
+};
+
 var cachify = exports.cachify = function (filename, options) {
   var tag_format;
   if (! options) options = {};