Align label mapping with imdb dataset

Hi there, your model is using a default label mapping. Accept this PR to align the label mapping with the `imdb` dataset this model was trained on. This will enable your model to be evaluated by [Hugging Face's automatic model evaluator](https://huggingface.co/spaces/autoevaluate/model-evaluator?dataset=imdb)

config.json

@@ -17,5 +17,13 @@
   "num_hidden_layers": 1,
   "pad_token_id": 0,
   "type_vocab_size": 2,
-  "vocab_size": 30522
-}
+  "vocab_size": 30522,
+  "label2id": {
+    "neg": 0,
+    "pos": 1
+  },
+  "id2label": {
+    "0": "neg",
+    "1": "pos"
+  }
+}