Add support for customizing environment variables using `python-dotenv`

[dwhan89]

Description:
Currently, there is no streamlined way to manage environment variables (such as OpenAI API key) within the project. To improve configuration management and user experience, it is probably worthwhile integrating python-dotenv (or something like that) for handling environment variables

---

Proposal

1. Add python-dotenv as a dependency in the project.
2. Create an editable .env.template file in the root directory.
3. Add an instruction on how to customize the .env.template file in the README.
4. Update the codebase to load environment variables from the .env file either automatically or manually.
5. Edit the .gitignore file to prevent users from committing the .env file to the remote repository.

Goals

• Simplifies configuration management.
• Improves workflows by enabling end-users to manage environment-specific variables easily.

[dwhan89]

Note: This is not a critical feature. I can create a pull request once it has been implemented.