Added ability to override the arguments

I needed to be able to use a different set of arguments. Updated to allow
for the args to be passed in the same way the src and dest are. Could
improve by comparing the two arrays instead of simply overriding them.

tasks/wkhtmltopdf.js

@@ -51,15 +51,24 @@ module.exports = function(grunt) {
           "Converting " + src + " -> " + dest
         );
 
-        // Launch PhantomJS.
-        helper.convert({
-          code: 90,
-          args: [
+        // default args
+        var args = [
           '--dpi', '96',        // workarround to wkhtmltopdf letter-spacing bug (see http://code.google.com/p/wkhtmltopdf/issues/detail?id=72)
           '--print-media-type', // Use @print media type
-              src,
-              dest
-            ],
+        ];
+
+        // overrides the args
+        if (file.args) {
+          args = file.args;
+        }
+
+        // adds the src and dest
+        args = args.concat([src, dest]);
+
+        // Launch wkhtmltopdf.
+        helper.convert({
+          code: 90,
+          args: args,
           done: function(err) {
             if (err) {
               grunt.log('>>>', err);