v2.0.0

Added

• Hooks into Jekyll::Assets if available
• Adds frozen string literal comment

Changed

• Uses built in Jekyll.env instead of ENV["JEKYLL_ENV"]
• Changes Jekyll::Zopfli::Compressor to a module and implements a compress_directory method
• Moves Jekyll::Zopfli::ZIPPABLE_EXTENSIONS into plugin config that can overwritten in the site config