add a data-uri function

as described in a comment on #775

lib/less/functions.js

@@ -1,3 +1,5 @@
+var fs = require('fs');
+
 (function (tree) {
 
 tree.functions = {
@@ -345,6 +347,18 @@ tree.functions = {
     },
     shade: function(color, amount) {
         return this.mix(this.rgb(0, 0, 0), color, amount);
+    },
+    "data-uri": function(mimetype, path) {
+        var data = fs.readFileSync(path.value);
+        mimetype = mimetype.value;
+
+        if (/;base64$/.test(mimetype)) {
+          data = (new Buffer(data)).toString('base64');
+        }
+
+        var contents = new(tree.Anonymous)('data:'+mimetype+','+data);
+
+        return new(tree.URL)(contents);
     }
 };