Add 'raw' tag and 'prepend' filter to Liquid processing.

Two additions, both via `liquid.patch.js`:
1. The Jekyll starter project makes use of the `prepend` filter in Liquid, but it's not available in the current version of liquid.js.  (Note: there's a fairly [recent commit](https://github.com/darthapo/liquid.js/commit/39d5cd3d1c74f7401efc3332e7441947fa2ea397) there that adds it and several other standard filters, but no new release.)
2. Support for `{% raw %}` tag.