Fixes#9
Add support for output via pipe `repo-to-text > myfile.txt`.
* Modify `save_repo_to_text` function in `repo_to_text/main.py` to write to stdout if `--stdout` is specified.
* Add `--stdout` argument to `argparse` in `repo_to_text/main.py`.
* Update `main` function in `repo_to_text/main.py` to handle the new `--stdout` argument.
* Update `README.md` to include instructions for using the new pipe output feature.
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