Added support for updating custom fields.

lib/src/entity.ts

@@ -0,0 +1,16 @@
+import { CustomField } from './customField'
+
+export interface Entity {
+    custom_fields?: CustomField[];
+    id: number;
+    type: EntityType;
+}
+
+// EntityType must be one of:
+// event, exercise, gear, healthsafety_report, incident,
+// incident.weather, member, person_involved, unit, activity
+//
+// Only the ones actively in use are implemented.
+export enum EntityType {
+    Member = 'member',
+}