Retry Capture on HTTP Error/Status

Sometimes we have temporary issues with HTTP - like the server timing out or in my case, fairly frequent 503 errors when Varnish plays up.

It'd be useful if I could make BackstopJS retry if the HTTP code wasn't 200 (and/or a collection of OK codes, including redirects perhaps and/or a customisable list of codes). Perhaps with a retry limit.

At the moment I get a fair emount of fails because either the reference or test image is just a 302 error page.

I'm happy to look into making a pull request if it's something you'd like included.