Allows the addition of arbitrary fields to $searchable_fields that you can satisfy with a function. Any field not also in DB are considered to have the Elasticsearch type string.
Allows the addition of arbitrary fields to $searchable_fields that you
can satisfy with a function. Any field not also in DB are considered to
have the Elasticsearch type string.

Also a quick fix to Searchable where the applicaiton was passing an
array to $is_callable, causing it to fail.

gordonbanderson/silverstripe-elastica,gordonbanderson/silverstripe-elastica