Drop Config and go back to kwargs on `validate`

[stefanvanburen]

We previously added the Config class to support alternative regex implementations, but we dropped that in #342. Now Config just has the single fail_fast option on it.

We ought to drop Config and go back to just adding a fail_fast kwarg parameter on validate, given that we don't expect to add other configuration options.

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Given that we've gone back and forth on the shape of the exported API, ideally we'd get both this and #361 implemented, and publish a 0.15.0 release with both breaking changes included.