Add sass compilation support

[facundoolano]

It would be useful to have build and serve convert .SCSS files to CSS. But I'm interested in this only as long as it doesn't require an extra component, so I'd favor golibsass over the alternatives that require the Dart executable, even if libsass is deprecated.

Note that together with this change we'd likely want to revisit the fact that conversion from source to taget formats (e.g. org and md to html) is now only done for templates (files headed by front matter). We'd likely want scss files to be converted without require them to be flagged as templates; perhaps we should do the same with markdown and org. (this would also point in the direction of decoupling org/md code coversion from liquid rendering, which sounds like the right thing to do)