Bugfix/increase tokens (#132)

* feat(api.ts): add support for OPENAI_MAX_TOKENS environment variable
feat(config.ts): add OPENAI_MAX_TOKENS to CONFIG_KEYS and configValidators

* 2.0.2

* 2.0.3

* docs(README.md): add GPT-4 usage instructions and mention the cost difference compared to GPT-3.5-turbo

---------

Co-authored-by: di-sukharev <dim.sukharev@gmail.com>

README.md

@@ -54,6 +54,18 @@ oc
 
 ## Features
 
+### Switch to GPT-4
+
+By default OpenCommit uses GPT-3.5-turbo (ChatGPT).
+
+You may switch to GPT-4 which performs better, but costs ~x15 times more 🤠
+
+```sh
+oc config set model=gpt-4
+```
+
+Make sure you do lowercase `gpt-4`.
+
 ### Preface commits with emoji 🤠
 
 [GitMoji](https://gitmoji.dev/) convention is used.