Add SASS support

[f0x52]

This pr adds some simple sass compilation using node-sass. I'm unsure how to write tests for this though, so that step from CONTRIBUTING is missing.

[Gioni06]

Hey. Nice idea and definitely something that a lot of projects require.

What makes this project different from other static site generators is its minimalistic approach. Therefore I would like to raise the argument against adding Sass support in the core of nanogen.

However I think it should document (with example) on how to manually compile the assets with a simple script or a taskrunner like gulp.

What do you think ?

[Gioni06]

@f0x52 You might want to check out Stasis. It's a very straight forward static site compiler that lets you write your templates in markdown just like nanogen. Also, it can compile all your assets (sass, less, scss) and lets your query your pages using GraphQL. If you are not looking for an enterprise-grade static site compiler like Gatsby or Hugo :)