Add Option to Specify Repository Branch

Currently, git2txt defaults to the repository’s default branch when converting its contents into a text file. However, many repositories maintain multiple branches (e.g., develop, feature/*, release/*) that users might want to analyze or document separately. The ability to specify a branch would provide greater flexibility and control over which version of the repository is processed.

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Introduce a new command-line option (e.g., --branch or -b) that allows users to specify the branch they wish to convert. For example:

```
git2txt username/repository --branch=develop
```

### Proposed Option Details:

* Option Name: --branch or -b
* Description: Specifies the branch of the repository to use for conversion.
* Default Behavior: If the option is not provided, the tool will continue to use the repository’s default branch.

```bash
# Specify the 'develop' branch
git2txt username/repository --branch=develop

# Using the short flag
git2txt username/repository -b feature/new-feature
```

Benefits:
* Flexibility: Users can choose which branch to convert, enabling analysis of different development stages.
* Efficiency: Facilitates working with repositories that have active development on multiple branches without needing to switch branches manually.
* Enhanced Functionality: Aligns git2txt with common Git workflows, making it more versatile for various use cases.