Huggingface tokenizer support

[melang982]

Adding Huggingface tokenizer support. This is useful for RWKV models that were trained with a custom tokenizer, especially since RWKV tokenizer training code is not available. Useful for experiments such as per-character tokenizer or custom datasets such as music, timeseries, rare languages etc

Checked with ai00_server and my trained from scratch RWKV model that uses BBPE HF tokenizer - it works 🎉

[cryscan]

Thanks! Actually tokenizer is not a that "hard dependency" that applications built upon web-rwkv would rely on -- they are always free to implement their own tokenizer. This is because web-rwkv's model APIs only see tokens, not text strings. I will review this after #24 is merged.