onAfter to support things like dumping chrome perf report

Since there was another ticket closed for an already supported use case I figured I'd open a request for onAfter for another use case that I'm not sure would be supported by putting cleanup at the start on the onReady/onBefore script.

I would love to use the [puppeteer trace](https://github.com/GoogleChrome/puppeteer/blob/v1.19.0/docs/api.md#class-tracing) to capture page load time reports along with the screenshot. AFAICT there's no way to do this with the existing Backstop API