Consider frontend plugins as a method for adding features to default templates

[isaacwhite]

@afischer points out that mkdocs has some nice features to allow for custom formats within documentation that include custom styles on the pages. It would be really powerful to add a new customization type that supported this pattern.

Could include something like the following properties, with our code highlighting logic is a proof-of-concept.

module.exports = {
  matcher: /regex or callback here/, // either some regex or a callback that returns an array of matches
  replacement: (match, captureGroupEtc) => {
    // this returns some HTML to put in the output in place of the matched text
  },
  scripts: // string or callback, these get injected into the page when matcher returns a nonempty array
  styles: // string or callback, these get injected into the page when matcher returns a nonempty
  domains: // optional, new domains to allow in csp for script/style additions
}