js.gz + css.gz handling

[matteofigus]

This should be the first part of #138

Basically, when static folder contains files with .css.gz or js.gz extensions,

• oc dev watcher handles it (forwards to the client a Content-Encoding:gzip header for given resource)
• s3 publish should save the ContentEncoding property to the S3 object (so that Cloudfront should recognise it and forward it)

[matteofigus]

Can you review @andyroyle ?

[andyroyle]

makes sense to me

[andyroyle]

published 0.20.5

[matteofigus]

👍