Is there a way to set site permissions? #376

Thanks for this great library!

My specific use case for altering site permissions is that I am testing a subscription workflow for push notifications, and I would like to test the user experience when the value of Notification.permission is "default", "granted", or "denied".

# in headless mode
browser.evaluate("Notification.permission") # => "denied"

# in non-headless mode
browser.evaluate("Notification.permission") # => "default"

I'm wondering if there is a way to set this value to "granted", or to modify site permissions in general. I could not find anything relevant to this in the README, the code, or the command line args.