support chatGPT model

When trying to set `gptcommit config set openai.model gpt-3.5-turbo`

after that, when using gptcommit, It shows the error 

```OpenAI error: This is a chat model and not supported in the v1/completions endpoint. Did you mean to use v1/chat/completions?```

reference: https://openai.com/blog/introducing-chatgpt-and-whisper-apis
