Make cfasync script attributes opt-in

[jblz]

Is your feature request related to a problem?

As mentioned here, there's no need to have these attributes in place for sites that don't use Cloudflare.

It should, therefore, be opt in. Since it's a breaking change, I'm adding it to the 3.0.0 milestone.

Describe the solution you'd like

Either a checkbox in the "Advanced" settings section or a filter to opt-in to this behavior (I'd lean toward the latter).

Describe alternatives you've considered

Additional context

[GaryJones]

+1 for the filter approach, and not having a new setting.

We could also make this opt-out for a 2.6.0 release, and then opt-in for the 3.0.0 release.