Better JS bundling

[sc0ttj]

1. Support multiple JS CDNs:

Allow user to set preferred CDN in site.yml:

• https://cdn.jsdelivr.net/preact@10.1.1/
• https://unpkg.com/preact@10.1.1/
• others?

2. Bundle the NPM deps defined in site.yml as a single JS file:

Reduce network requests, while keeping things cache-able, by bundling the main site JS deps as a single file at build time:

• site deps: use curl at build time to grab the JS, put it into assets/js/site.js

If a page has its own, separate list of deps, bundle them into a separate file:

• page deps: use curl at build time to grab the JS, put it into assets/js/${page_slug}.js