Make gz suffix optional

I'm using broccoli-gzip with travis s3 deploy. It has an option to set content encoding header if you compress files before uploading them (http://docs.travis-ci.com/user/deployment/s3/#Setting-Content-Encoding-header)

In order for the links to work, I need to keep the original file extensions (not add the gz suffix).

I wasn't 100% what to name the flag but this is how they refer to it in the gzip command line help. Feel free to rename to something better.