Feature: Calculate usage info for streamed responses

[jschuur]

Streamed responses don't include usage info in the response. Would have to calculate this via [tiktoken])(https://github.com/dqbd/tiktoken) e.g., but that would be a big dependency for the project.

Alternatively, count chunks manually for rough estimate (more info).

• estimate usage
• mark usage as estimated in usage history (assumes Feature: More detailed usage tracking #20)
• update Readme section when implemented

[jschuur]

More lightweight, JS solutions:

https://www.npmjs.com/package/gpt-tokens
https://www.npmjs.com/package/gpt-tokenizer

[jschuur]

Usage stats for streaming now supported (at least on completion):

https://cookbook.openai.com/examples/how_to_stream_completions#4-how-to-get-token-usage-data-for-streamed-chat-completion-response