Provide a hook to alter the parameters

Patch is attached to implement a hook that allows the parameters to be altered.

Updating to 2.x-dev.

Thank you for you submission. Converted patch to merge request.

Now all this needs is tests.

Thanks for the MR! Also, to weigh in on the discussion about alter hooks vs other ways of changing the parameters: we want the way to alter this to be as lightweight as possible. Cached pages should be served with minimal overhead.

When getting the defined parameters, we can't even rely on everything in the container yet. The module handler works, but I still had to force it to load all .module files. Using the event system could become problematic, same goes for plugins. Dependency injection could easily lead to a circular reference (found that out the hard way while trying to do fancy stuff in my implementation of the hook).

Overall, hooks still have their place in the Drupal universe and I don't see that changing anytime soon.