Remove Sass and replace with Pleeease

This commit removes Sass support from the project and adds Pleeease as a
CSS post-processor.

Preprocessors do have a couple of problems:
* CSS is meant to be declarative. Preprocessors add a lot of functions
to it that are not easy to understand and require higher level thinking.
Logicless-ness makes CSS simple.
* Specificity is often problematic (e.g. with deep nesting).
* People that are not used to one or the other preprocessor are left
behind. Everyone knows CSS.

Reference: #123.
Sorry, haven‘t seen the reference in package.json :)