Add support for NEL

[jacobbednarz]

Network Error Logging is becoming a thing via the [Reporting API] so it would be good to add support for it. Ideally, we would:

• Rename this repository to be something more generic
• Have support for CSP and NEL via an extensible interface
• Make it easy to add reporting API endpoints in the future

[frenchvandal]

Nice, I was looking for a go alternative to that php script.

What about Expect-CT? Do you plan to add support too?

[jacobbednarz]

Eventually, it's definitely possible. The desire for NEL comes from our internal use of this tool and it hasn't been a must-have just yet but if you'd like to open a PR, I'd gladly review it.

The only restriction is that we'd want a separate endpoint (/expect-ct or similar) for the CT reports so we'd also need to shuffle the CSP endpoint to /csp.