Have you searched for similar issues before submitting this one?
Yes

Is this a bug, question or feature request?
Feature

Describe the issue you encountered:
Our search result page has links that allow users to change the sort order, and to limit the results to a single post type. These links go to URLs with query parameters like ?post_type=page&orderby=date&s=hello. When we add the facet widget, the terms in the widget are linked to URLs with query parameters like ?s=hello&filter_foo=bar. The post_type and orderby params are not retained, so the post type limitation and sort order are lost when the user clicks on a facet term. We want these to be maintained.

I would propose adding a filter that could be applied here:

So at line 451, something like:

$query_string = apply_filters( 'ep_facet_query_string', $query_string );

Happy to make a pull request for this.