support jsonify filter

Now jekyll supports automatically loading yaml files under `_data/`  directory. To use the data in front-end to build rich client apps, there should be an easy way to convert the data into JavaScript.

The `jsonify` filter fulfills that purpose.

For example, following code exports `site.data.projects` for front-end usage.

``` javascript
window.projects = {{ site.data.projects | jsonify }}
```