Support generation of HAR for entire test run

Hello, Love the project - fantastic idea.

Would it possible to modify the API such that a HAR file can be generated after the Chromeless run has completed?

There's a lot of beneficial data in the HAR file - e.g. how long an HTTP request/response cycle took? The format has a full timing based breakdown as well.

Normally the HAR file can be saved via the Chrome web inspector so I'm not sure if that's possible with headless Chrome.

Also not sure if this issue is related to #91 ?