Add support for to_javascripts url property

1. Unprotect make_url.
2. Add simple delegator for model.url.

This will allow
```mustache
---
custom: some.js
---
{{# page.custom?to_javascripts }}
   <script type="text/javascript" src="{{ url }}"></script>
{{/ page.custom?to_javascripts }}
```
to render out
```html
<script type="text/javascript" src="/assets/javascripts/some-...hash....js"></script>
```
using the correct fingerprinted javascript file url.