Feature: Add the ability to log in to pages

[fpapado]

Some flows might need the user to be logged in (via a Cookie, typically?) to get to the page we need to test.

It would be nice to have the option to do that!

I am not 100% settled on how we will achieve this.
My first thought was to add an option to read a cookie from a file.
However, that seems overly specific.

Another option would be having users to pass in a Puppeteer Page object by themselves. They could log in before hand, using selectors.

A middle ground would be to add an onBeforeAssert hook, that lets the user run scripts with the Page context. Would that be better? Worse? Horrible? Let's find out! :)

Prior art

BackstopJS has a setting cookies option, that allows loading cookies from a file. Maybe that would work as well?