[s3sync] Add SYNCEXTRA environment variable

[sven-codeculture]

Make it possible to add extra parameters to the sync_cmd (such as --cache-control, for creating cache headers for all uploaded files)

This would be really helpful since i sync an CDN using this project

[vladgh]

This would be a nice addition, thanks. But before I merge it would you please address the comments I made.

[vladgh]

Thanks.

[vladgh]

Let me know how it works in production.

[sven-codeculture]

I am already using it in Production. I've currently replaced the file in an own docker image based on yours.
By adding SYNCEXTRA: --cache-control max-age=2419200,public to my docker-compose for the sync command, every newly uploaded file gets the header "cache-control: max-age=2419200,public" which i require for pagespeed on the CDN.
Unfortunatly it only works for newly uploaded files, already existing files needs to be modifyed manually.
Its also possible to add other metadata for newly uploaded files using this, according to the sync documentation.