Implement worker

Closes #2 (closed)

.arkindex.yml

@@ -9,3 +9,16 @@ workers:
     type: data-extract
     docker:
       build: Dockerfile
+    user_configuration:
+      train_folder_id:
+        type: string
+        title: ID of the training folder on Arkindex
+        required: true
+      validation_folder_id:
+        type: string
+        title: ID of the validation folder on Arkindex
+        required: true
+      test_folder_id:
+        type: string
+        title: ID of the testing folder on Arkindex
+        required: true