Support opt-in multi-page main frame navigations

[JackSpiece]

Summary

• add an opt-in allowMultiPage option for main-frame scheduled or requested navigations
• create the new HAR page when the next main-document request starts so page timing uses that request
• keep the default flat behavior unchanged and map responses by the entry pageref

Fixes #65
Gitpay task: https://gitpay.me/#/task/348

Validation

• npm test
• npm run lint
• git diff --check