Pass configured options to Uglifier

The [defaults in `Jekyll::Assets::Config`][1] include some
configuration for Uglifier - e.g. enabling Harmony mode so you
get ES6 support.

However, we don't actually do anything with this configuration
as of 218f5ee (see
`Jekyll::Assets::Env#enable_compression!` which is removed).
This leads to issues like #511.

This starts passing that configuration into `Uglify`, our
`Sprockets::UglifierCompressor` subclass, in
`lib/jekyll/assets/compressors/uglify.rb`. The
[in-code documentation for Sprockets][2] notes that you can supply
an instantiated, configured instance rather than a class and it'll
be used.

Fixes #511.

[1]: https://github.com/envygeeks/jekyll-assets/blob/master/lib/jekyll/assets/config.rb#L24
[2]: https://github.com/rails/sprockets/blob/master/lib/sprockets/uglifier_compressor.rb#L16

lib/jekyll/assets/compressors/uglify.rb

@@ -16,8 +16,14 @@ def call(input)
         end
       end
 
-      # rubocop:disable Metrics/LineLength
-      Sprockets.register_compressor "application/javascript", :assets_uglify, Uglify
+      Hook.register :env, :before_init, priority: 1 do |e|
+        uglifier_config = e.asset_config[:compressors][:uglifier].symbolize_keys
+
+        Sprockets.register_compressor "application/javascript",
+          :assets_uglify,
+          Uglify.new(uglifier_config)
+      end
+
       Hook.register :env, :after_init, priority: 3 do |e|
         e.js_compressor = nil
         next unless e.asset_config[:compression]