Add token_indexer tests

combo/data/__init__.py

combo/data/token_indexers/__init__.py

combo/data/token_indexers/pretrained_transformer_indexer.py

combo/data/token_indexers/single_id_token_indexer.py

combo/data/token_indexers/token_features_indexer.py

combo/data/tokenizers/token.py

combo/example.ipynb

tests/data/test_vocabulary.py

tests/data/token_indexers/__init__.py

tests/data/token_indexers/test_single_id_token_indexer.py

tests/data/token_indexers/test_token_characters_indexer.py

tests/data/token_indexers/test_token_features_indexer.py

tests/fixtures/large_vocab/.lock

tests/fixtures/large_vocab/deprel_labels.txt

tests/fixtures/large_vocab/feats_labels.txt

tests/fixtures/large_vocab/lemma_characters.txt

tests/fixtures/large_vocab/non_padded_namespaces.txt

tests/fixtures/large_vocab/token_characters.txt

tests/fixtures/large_vocab/tokens.txt

tests/fixtures/large_vocab/upostag_labels.txt