Add support for page.setExtraHTTPHeaders()

[tollmanz]

Users should be able to set any headers they want when making a request and recording a timeline. Let's add support for page.setExtraHTTPHeaders() so these headers can be customized.

This is a fairly straight forward task; however, we need to decide on a good UI for adding headers via the command line. setExtraHTTPHeaders takes an object, which is a pain for a user to define via the CLI. I think we might want something like:

# cURL style
--set-extra-http-headers "referer: https://bar.com" --set-extra-http-headers "origin: foo.com" 

# List style
--set-extra-http-headers "referer: https://bar.com,origin: foo.com"