LSTM-CRF model with static context embedding

As we are going to pushing the version with the transformers package (by huggingface) integrated, it is necessary to release the current version as a base. We try to make sure everyone who uses this repo can get back to this version if he/she wants to start from there.

The major functionality:

1. State-of-the-art BiLSTM-CRF architecture
2. It allows users to incorporate static context embedding.
3. Users can save the model and use it for other downstream tasks.