This is a rewrite of the plugin with breaking changes to the options
object.

Please refer to the upgrade document[0] and README[1] for more
information.

This plugin now uses version 3 of uglify-js, or uglify-es.

[0] https://github.com/borisovg/metalsmith-uglify/blob/master/1to2.md
[1] https://github.com/borisovg/metalsmith-uglify/blob/master/README.md