Preserve sessions between scenarios in specific use cases.

[clfer]

Hello

We want to test a web application that is logged in only.
The way Behat handle tests isolation, require that we repeat the login process for each and every one of our scenarios.
The login process accounts for almost 20% of our testing time.

I suggest a new tag option to preserve sessions between scenarios.
This should not be the default behavior and only be used in specific use cases.