Add BERTweet and PhoBERT
I'd like to add BERTweet and PhoBERT to the huggingface's transformers library. 
Users can now use these models directly from transformers. E.g: 

    tokenizer = BertweetTokenizer.from_pretrained("vinai/bertweet-base")
    bertweet = BertweetModel.from_pretrained("vinai/bertweet-base")