Config option to send 404.html(.gz) files to browsers for static sites

[eliotsykes]

rack-zippy will pass any request paths it can't fulfil down the rack middleware stack.

In a static site I've got a config.ru like this to handle the 404s:

require 'rack-zippy'
require 'rack/contrib/not_found'

asset_root = 'dist'
use Rack::Zippy::AssetServer, asset_root
run Rack::NotFound.new "#{asset_root}/404.html"

Unfortunately Rack::NotFound doesn't serve 404.html.gz. I'd like for rack-zippy to offer this as an option (off by default), where static sites can ask rack-zippy to handle 404 Not Found errors by serving 404.html.gz (if present and browser is gzip-capable), otherwise 404.html.