added sassify and scssify filters for converting sass and scss strings to css

This branch creates two filters that are very similar to markdownify. Currently, there is no way to manually convert Sass or Scss. A popular use case would be inlining critical, above-the-fold CSS, something that is [very important for performance](https://developers.google.com/speed/docs/insights/OptimizeCSSDelivery). This branch would allow you to do something like this:

```
<head>
{% capture include_to_scssify %}
{% include critical_css.scss %}
{% endcapture %}
{{ include_to_scssify | scssify }}
</head>
```

I can update the documentation if this gets the thumbs up. I just wanted to make sure I didn't get a "this can be a plugin!" response before I put time into updating the docs.

Thanks!