This is similar to toastdriven#360, but allows you to pass options per-facet, and is not SOLR-specific. It includes test/documentation updates, and implementations for SOLR and ElasticSearch (Whoosh does not support faceting anyway).
Added support for sending facet options to the backend
Thanks for the pull request - this looks like a good idea to me, particularly for avoiding more kwargs on search().
Handle "global" and "facet_filter" options for ElasticSearch
Some useful facet options in ES are applied at the facet level, not the "terms" level. Specifically, the "Scope" and "Facet Filter" sections of http://www.elasticsearch.org/guide/reference/api/search/facets/.
Fixed up unit tests