Add Cache-Control header

[benoitchantre]

Prevent changes by proxies.

Closes #12.

[TheLastCicada]

Interesting thought to add no-transform here. Not sure if no-transform is the default I always want however. What benefit do you have in mind with this? What case would you make that we should prevent proxies from applying additional compression or other transformations to these files? I'm thinking of things like Cloudflare which can implement additional image compression or compress js or css files with brotli on the way through their proxy and I wouldn't necessarily want to prevent that.

[jeffpaul]

@benoitchantre any responses to the questions in the prior comment from @TheLastCicada?

[benoitchantre]

@TheLastCicada @jeffpaul Sorry for the late reply. I think my mind was focused on security. It skipped optimizations that can be provided by a CDN. For this reason, this header should be only added in specific cases (not by default).