Allow external fonts to PPTX

[tjinauyeung]

This PR adds an addFonts method which enables external fonts to be added to the PPTX.

How to test

1. Pull changes
2. Bundle lib - npm run ship
3. Run demos - cd demos; node ./browser_server.mjs
4. Try out API

Example:

pptx.addFonts([
  { 
    name: "Roboto",
    styles: [
      { name: "regular", path: "http://localhost:8080/roboto-regular.fntdata" },
      { name: "bold", path: "http://localhost:8080/roboto-bold.fntdata" }
    ]
  }
])

In order to pass a valid path to .fntdata files you can serve them locally. Note: consider CORS when serving.

Follow up

• Build JS assets to dist
• Add new tag
• Include in pptx-exporter

[zarkone]

updated "how to test"

[pastafari]

oooff this index thingy is not pretty!