jekyll-gzip

Fixed

• Replacing files with gzipped version wasn't working, fixed thanks to k0nserv in #6

Added

• Adds setting to replace original files with gzipped for serving from AWS S3

Added

• Adds frozen string literal comments
• Tries to hook into Jekyll::Assets if available

Changed

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