feat: model configuration to support different models (#183)

Co-authored-by: hiroki osame <hiroki.osame@gmail.com>

README.md

@@ -180,6 +180,12 @@ To clear the proxy option, you can use the command (note the empty value after t
 aicommits config set proxy=
 ```
 
+#### model
+
+Default: `gpt-3.5-turbo`
+
+The Chat Completions (`/v1/chat/completions`) model to use. Consult the list of models available in the [OpenAI Documentation](https://platform.openai.com/docs/models/model-endpoint-compatibility).
+
 ## How it works
 
 This CLI tool runs `git diff` to grab all your latest code changes, sends them to OpenAI's GPT-3, then returns the AI generated commit message.