Support token-based chunking in chunk_by_title using tiktoken

Currently, `chunk_by_title` in `unstructured.chunking.title` uses `max_characters` for chunking, but I need it to support token-based chunking (e.g., 512 tokens per chunk) using `tiktoken`. The current implementation isn't ideal for token-based models like OpenAI GPT, which rely on token limits rather than character limits. How can we modify this to chunk by tokens instead of characters? Any guidance or suggestions to solve this would be appreciated!