Halftone-converter

No features changed
- Refactor: Bisection method
- Update: README & LICENSE

- Fix: unclear messages when an empty strings is given as a filename
- Fix: creating needless directories in case of the paths including parent directory terms

- Add `-H`, `--allow-huge` option
- Add `-K`, `--keep-all` option
- Add `--key` option synonym
- Performance optimization

- The first release