introducing "gzip" strategy support

• "strategy": ["gzip"] is now an option for CDN targets
  • this will upload GZIP-deflated versions of all files
  • if you need to support ancient browsers, then you will need separate targets for "clone" and "gzip" and a way to direct compatible browsers to the "gzip" CDN

coming soon

• automatic correction of remote headers i.e. if GZIP-deflated resources lack Content-Encoding: gzip
• better documentation and JSON Schema for .cdn-sync.json