fix: export MigrationModel and MigrationStore inferfaces (#39)

Author: lukashass

src/index.ts

@@ -1,5 +1,6 @@
 import { Migration, MigrationContext } from './Migration';
+import { MigrationModel, MigrationStore } from './MigrationStore';
 import { MongoMigrationStore } from './MongoMigrationStore';
 import up from './up';
 
-export { Migration, MigrationContext, MongoMigrationStore, up };
+export { Migration, MigrationContext, MigrationModel, MigrationStore, MongoMigrationStore, up };