Opening external anchors using rel="noopener"

[laurentpellegrino]

For performance and security reasons, it would be really convenient to add an option that allows specifying rel="noopener" for external links:

https://developers.google.com/web/tools/lighthouse/audits/noopener

Especially, when the link target is specified in configuration: 47e7e1c.

[doowb]

Thanks for the PR #293 to handle this. As mentioned in #291 if that feature is available then I think the rel="noopener" should be handled with that feature to allow users more control over how they handle it themselves.