Allow theme to load Materialize assets over CDN

Problem/Motivation

Allow theme to load Materialize assets over CDN

Proposed resolution

Implementing CDN could be as easy as defining a theme library which contains the Materialize assets over CDN

Example:

materialize-cdn:
  css:
    base:
      //fonts.googleapis.com/icon?family=Material+Icons: { external: true }
      //cdn.jsdelivr.net/npm/@materializecss/materialize@2.1.1/dist/css/materialize.min.css: { external: true }
  js:
    //cdn.jsdelivr.net/npm/@materializecss/materialize@2.1.1/dist/js/materialize.min.js: { external: true }

Next steps would be to introduce override libraries for component based extended functionality