Support --cookie-jar for multiple Hurl files

[jcamiel]

As of Hurl 4.2.0, we only support --cookie-jar if Hurl is run with exactly one Hurl file:

$ hurl --cookie-jar build/cookies.txt tests_ok/cookie_jar.hurl tests_ok/cookie_jar.hurl
error: Only save cookies for a unique session

I don't remember why we've done this but I think we can remove this restriction. Netscape cookie format support comment so we can indicate, in comment, from which file the cookies are dumped.