Request: Generation of x509/TLS certificate+key, chain signing

[twitchyliquid64]

Would be good to have some copypasta for generation of TLS certificates, especially certificates signed in a chain.

I have done this myself (twitchyliquid64/certlib), but don't pretend to be a crypto expert so I am hesitant to make a PR - I will probably have some problems.

[gtank]

I'm open to discussion on this, but mostly consider TLS and certificates to be too complex/situational for unambiguous advice. I've tried before and failed at writing something that works out of the box for everything people need to do with certificates in production. I suspect devs would be better served by existing tools like https://godoc.org/rsc.io/letsencrypt or https://github.com/cloudflare/cfssl than some fairly independent generation functions.