Provides the ability to import JS and CSS files from CDNs

[juanpedromoreno]

Fixes #84

This PR provides the ability to include cdn imports across the different layouts in this way:

micrositeCDNDirectives := CdnDirectives(
  jsList = List(
    "https://cdnjs.cloudflare.com/ajax/libs/ag-grid/7.0.2/ag-grid.min.js",
    "https://cdnjs.cloudflare.com/ajax/libs/ajaxify/6.6.0/ajaxify.min.js"
  ),
  cssList = List(
    "https://cdnjs.cloudflare.com/ajax/libs/animate.css/3.5.2/animate.css",
    "https://cdnjs.cloudflare.com/ajax/libs/cssgram/0.1.12/1977.min.css",
    "https://cdnjs.cloudflare.com/ajax/libs/cssgram/0.1.12/brooklyn.css"
  )
)

Please, @jdesiloniz could you review? Thanks!

[jdesiloniz]

LGTM!!!