Enable custom headers for sirv-cli

[danitrod]

In a really specific application I would like to quickly setup a server with sirv-cli, but I need some specific headers to be sent with my resources (namely COOP and COEP). I was thinking maybe a --custom-header <name>:<value> option would be an interesting addition. Any thoughts? Happy to help contributing code if needed.

[adamdbradley]

I have this same requirement and would love to see this feature.

[MarkusKramer]

Would be good to set correct CSP headers during development.