implement new feature - function calling

[smoking-caterpillar]

With the release of gpt-3.5-turbo-0613 and gpt-4-0613, OpenAI implemented function calling which now makes it possible to call python functions via chat responses.
It can also work with GPT 3.5 for anyone not on the GPT-4 beta.
More info - https://platform.openai.com/docs/guides/gpt/function-calling

[smoking-caterpillar]

This could be used to save files to the filesystem

[Meathelix1]

This could be used to save files to the filesystem

You can do anything with just function calling now. I would say one of the best things OpenAI has done.

[NoCLin]

After my testing, implementing chat_to_files using function calling is very accurate.

I'm planning to submit a PR these days.

Here is the PR: #94

Can I get your review?

@AntonOsika @patillacode