add support for tracking generated files/content

Sometimes we need to know which assets were generated by a given sacy
block (for example to retrieve jsx components to pre-render on the
server).

In this case the smarty template can call asset_compile like this:

```
{asset_compile block_ref="react-components"}
  <script type="text/x-jsx" src="/jslib/a_component.jsx"></script>
  ...
{/asset_compile}
```

Sacy will assign the generated assets into
`$smarty.sacy.rendered_assets`:

[
  'react-components' => [
    // src relative to the document root
    ['type' => 'file', 'src' => '/assetcache/components-d141caabeb4e87ff2bd021da49e7305c-content.js'],
    ['type' => 'string', 'content' => '...rendered content...'],
  ],
  ...
]