Load external npm modules

[dfosco]

Would be great to be able to load external npm modules to get "third-party" tachyons modules or even additional libs into the same bundle.

Something like this, perhaps:

{
  "typeScale": [
    3, 2.25, 1.5, 1.25, 1, 0.875
  ],
  "spacing": {
    "root": 8,
    "ratio": 2,
    "steps": 6
  },
  "external": {
    "npm@animate-css": "^3.5.2"
  }
}

[johno]

This sounds great, we could also allow the user to determine if they want the classes to also have media queries added. Additionally the could potentially select which classes they want to bring in. So if the only wanted to bring in a subset of animate-css they could pass a class array:

"external": {
  "npm@animate-css": {
    "version": "^1.2.3",
    "mq": true,
    "classes": ["class-a", "class-b", "class-c"]
  }
}