User API for non-local assets

[mortenpi]

Adds an exported asset function that can be used to include non-local assets in the HTML:

makedocs(
  format = Documenter.HTML(
    assets = [
      # Standard local asset
      "assets/extra_styles.css",
      # Standard remote asset (extension used to determine that class = :js)
      asset("https://example.com/jslibrary.js"),
      # Setting asset class manually, since it can't be determined manually
      asset("https://example.com/fonts", class = :css),
      # Same as above, but for a local asset
      asset("asset/foo.script", class=:js, islocal=true),
    ]
  )
)

A practical example would be to e.g. add additional fonts via CDN:

assets = [
  asset("https://fonts.googleapis.com/css?family=Nanum+Brush+Script&display=swap", class=:css)
]

Towards #982