Marx v4.0.0

• Move to using CSS with PostCSS and drop the dependence on SCSS and Stylus
• Update all NPM dependencies
• Remove SCSS and Stylus from Gulp build script
• Update README.md
• Move stylesheet files to /src
• Remove $link-hover-color which was unused
• Use CSS variables which allows for overrides.

<link  href="https://unpkg.com/marx-css/css/marx.min.css" rel="stylesheet">
<style>
:root {
  --primary: #008512;
  --link-color: #1a9f60;
}
</style>

Refined and updated all of the SCSS and Stylus!