Cache id alterations

Problem/Motivation

In case a site-owner does modifications to the template and for instance adds a language switcher inside the popup, the data is cached per language on the first request causing the all subsequent visits to be redirected to the page of the first cached visit.

By being able to perform alterations on the cache-id it would be possible to make needed distinctions, like in the given case based on the route.

Proposed resolution

Implement a hook to make modifications to the cache id.

API changes

Document the hook to alter the cache id.