Spignite
Spignite works in Javascript; and uses existing, mature, libraries. In a nutshell, Spignite is a generic file processor, applied to a special case of building web sites. Simple file processing is the core concept Spignite is built upon.
All this makes Spignite easy to use and extend (it's all just Javascript after all) and - quite fast.
Works with HTML, Markdown, Nunjucks, Pug, SASS... And yes, you will probably need to use some JavaScript code :)
Nothing more, nothing less.