Add `params.initial_prompt` to set an initial prompt string

[sindresorhus]

It would be useful if the API had an initial_prompt parameter to set a string as the initial prompt. The OpenAI docs recommend always setting it to fix punctation, so it's quite a common need: https://platform.openai.com/docs/guides/speech-to-text/prompting

I'm aware I could already do something like this, but I don't think it makes sense for everyone to re-implement that logic.

[jhen0409]

Agree, that option will be very useful!

I did the same in https://github.com/mybigday/whisper.rn/pull/8, I think it can be built directly into the whisper.cpp. Maybe I will try to send a PR later.

[User1231300]

I also would appreciate this