Add event to allow altering filter parameters before processing

@marckwee, thank you for the contribution! This sounds like a great idea to me!

I committed your code to a merge request to make it easier for me to review. I took a very quick look at your patch: the changes seem clear to me, but I haven't yet had a chance to run the changed code... I may have more feedback when I do.

We should add a change record to assist people upgrading from 8.x-7.x

I didn't see any automated tests in your patch... we prefer to accept merge requests that have passing automated tests. Automated tests ultimately benefit you, because they ensure that future changes to this module (i.e.: by other people) will not break the functionality that your site depends on!