Halftone-converter

- Add `-v` option

- Added the ability to read image files from standard input by passing '-' as an argument.
- Introduced a new command-line option `--stdout` to direct the output to standard output, which is mutually exclusive with the `--directory` option.
- All system messages, including non-error related information, are now redirected to `stderr`. This ensures that system messages do not interfere with the actual output of the program, especially when using `--stdout`.

- Documentation update

- Documentation update

- Documentation update
- Refactor codes
- Relax requirements spec

- Organized under the package: `halftonecv`.
- Script entry point added: `halftonecv`.
- Module entry point included: `python3 -m halftonecv`.
- Blur options updated to: `[-b [{box,gaussian}]] [-B PX]`.
- Features resampling options: `[-F {nearest,linear,lanczos2,lanczos3,spline36}]`.
- Show progress bars
- Fixed exit code
- Now published on PyPI under: [`halftone-converter`](https://pypi.org/project/halftone-converter/)
- Documentation has been updated.
- Support discontinued for Python 3.7 and Python 3.8.

Breaking changes ⚠️

- The default resampler has been updated - it's now `linear`, changed from `nearest`. Additionally, blur is disabled by default. To achieve the same results as the previous version, try options `--blur --resample nearest`.
- The program entry has been changed: Please use `halftonecv` or `python3 -m halftonecv`.