Add site.asset() helper for inlining assets

Probably not the most elegant way of doing it but this allows for something like:

```html
<script><%- site.asset('/js/foo.js') %></script>
```

or

```html
<style><%- site.asset('/css/bar.css') %></style>
```

... for when you want the assets to be inlined (like, oh I dunno, an error page where all the essential scripts and styles should be on the page itself rather than in separate files that may fail to load)