Introduce a deliberate option for onhashchange-based routing, fix #1734

[pygy]

trafficstars

WIP, pending discussing in #2179

Motivation and Context

See #1734

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Documentation change

Checklist:

• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.
• [ ] I have updated docs/change-log.md

[StephanHoyer]

@pygy do you want to rebase this?

[dead-claudia]

@pygy Did you forget to rebase or similar?

[pygy]

@dead-claudia no idea, the review request was sent by accident, I don't remember touching this issue in the recent weeks...

This is outdated, and irrelevant in the current context (IE is no more, and you revamped the router code).