Add option to persist cookies across VU iterations

[robingustafsson]

Currently k6 clears all cookies when a VU iteration ends, before the next one begins [1]. We should make this configurable (session cookies should probably still be discarded though).

Something along the lines of:
k6 run --persist-cookies script.js

In-script option:

export let options = {
    persistCookies: true|false
};

[1] - https://github.com/loadimpact/k6/blob/master/js/runner.go#L319

[na--]

As we briefly discussed yesterday, maybe the actual option name should be something like --no-cookies-reset, so we don't add a --yes-something option to the current mix of mostly --no-something options. Just adding it here so we don't forget to consider it when implementing this.