Lazy-load storage plugins to avoid side effects on import

lib/natural/util/storage/StorageBackend.js

@@ -20,12 +20,6 @@ OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 THE SOFTWARE.
 */
 
-const PostgresPlugin = require('./Postgres')
-const MongoDBPlugin = require('./MongoDB')
-const RedisPlugin = require('./Redis')
-const MemcachedPlugin = require('./Memcached')
-const FilePlugin = require('./File')
-
 // Enum for different types of storage backends that can be used
 const STORAGE_TYPES = {
   POSTGRES: 'POSTGRES',
@@ -49,6 +43,8 @@ class StorageBackend {
   }
 
   // Sets the storage type. Valid values are POSTGRES, REDIS, MONGODB, MEMCACHED, FILE
+  // Storage plugins are loaded on demand to avoid pulling in database drivers
+  // and their dependencies for users who only need NLP features.
   async setStorageType (storageType) {
     if (!storageType) {
       throw new Error('No storage type specified')
@@ -58,22 +54,32 @@ class StorageBackend {
     }
     this.storageType = storageType
     switch (storageType) {
-      case STORAGE_TYPES.POSTGRES:
+      case STORAGE_TYPES.POSTGRES: {
+        const PostgresPlugin = require('./Postgres')
         this.client = new PostgresPlugin()
         await this.client.configPostgres()
         break
-      case STORAGE_TYPES.REDIS:
+      }
+      case STORAGE_TYPES.REDIS: {
+        const RedisPlugin = require('./Redis')
         this.client = new RedisPlugin()
         break
-      case STORAGE_TYPES.MONGODB:
+      }
+      case STORAGE_TYPES.MONGODB: {
+        const MongoDBPlugin = require('./MongoDB')
         this.client = new MongoDBPlugin()
         break
-      case STORAGE_TYPES.MEMCACHED:
+      }
+      case STORAGE_TYPES.MEMCACHED: {
+        const MemcachedPlugin = require('./Memcached')
         this.client = new MemcachedPlugin()
         break
-      case STORAGE_TYPES.FILE:
+      }
+      case STORAGE_TYPES.FILE: {
+        const FilePlugin = require('./File')
         this.client = new FilePlugin()
         break
+      }
       default:
         throw new Error('Invalid storage type')
     }