Support upcoming prefetch-src directive

[Kiechlus]

https://w3c.github.io/webappsec-csp/#directive-prefetch-src
https://bugs.chromium.org/p/chromium/issues/detail?id=801561
w3c/webappsec-csp#107

[Evan Hahn]

Thanks for reporting this! Working on it now. I'll get to it in the next couple of weeks.

If someone wants to beat me to it and make a pull request, I think the place to look is in config.json. Adding a key to the directives list like 'prefix-src': { type: 'sourceList' } is a good place to start.

[Evan Hahn]

This has been released in helmet-csp@2.7.0 and helmet@3.10.0. Thanks for reporting!