make breadcrumbs referenceField an array

modules/islandora_breadcrumbs/config/install/islandora_breadcrumbs.breadcrumbs.yml

@@ -1,6 +1,7 @@
 maxDepth: -1
 includeSelf: FALSE
-referenceField: field_member_of
+referenceFields: 
+  - field_member_of
 dependencies:
   module:
     - islandora

modules/islandora_breadcrumbs/config/schema/islandora_breadcrumbs.schema.yml

@@ -7,6 +7,9 @@ islandora_breadcrumbs.breadcrumbs:
     includeSelf:
       type: boolean
       label: 'Include Self'
-    referenceField:
-      type: string
-      label: 'Reference Field'
+    referenceFields:
+      type: sequence
+      label: 'Reference Fields'
+      sequence:
+        type: string
+        label: 'Reference Field'

modules/islandora_breadcrumbs/islandora_breadcrumbs.install

@@ -0,0 +1,18 @@
+<?php
+
+/**
+ * @file
+ * Install/update hook implementations.
+ */
+
+/**
+ * Update referenceField config to referenceFields.
+ */
+function islandora_breadcrumbs_update_8001() {
+  $config_factory = \Drupal::configFactory();
+  $config = $config_factory->getEditable('islandora_breadcrumbs.breadcrumbs');
+  $config->set('referenceFields', [$config->get('referenceField')]);
+  $config->clear('referenceField');
+  $config->save();
+  return "Updated referenceFields config.";
+}

modules/islandora_breadcrumbs/src/IslandoraBreadcrumbBuilder.php

@@ -78,7 +78,7 @@ class IslandoraBreadcrumbBuilder implements BreadcrumbBuilderInterface {
     $breadcrumb = new Breadcrumb();
     $breadcrumb->addLink(Link::createFromRoute($this->t('Home'), '<front>'));
 
-    $chain = array_reverse($this->utils->findAncestors($node, [$this->config->get('referenceField')], $this->config->get('maxDepth')));
+    $chain = array_reverse($this->utils->findAncestors($node, $this->config->get('referenceFields'), $this->config->get('maxDepth')));
 
     // XXX: Handle a looping breadcrumb scenario by filtering the present
     // node out and then optionally re-adding it after if set to do so.