convert cnn data

python textsum_data_convert.py \
  --command text_to_vocabulary \
  --in_directories cnn/stories \
  --out_files cnn-vocab

python textsum_data_convert.py \
  --command text_to_binary \
  --in_directories cnn/stories \
  --out_files cnn-train.bin,cnn-validation.bin,cnn-test.bin \
  --split 0.8,0.15,0.05