Add an exclude option for aggregations

[kevinfodness]

A common use case is to include items in search results that have a certain post type (e.g., page) or term that we don't want to appear as selectable in the aggregation. Add an exclude parameter when registering aggregations that allows exclusion by key (for example, pages would still show up in search results, but the "Page" bucket wouldn't appear under the post type aggregation list in checkboxes etc).