* Implement warning diagnostic flags for RGBFIX
* `-m/--mbc-type help` prints to stdout
* Support `-m list` as well as `-m help`
* Make invalid `rgbfix -l` characters a fatal error, like other invalid CLI arguments
* Refactor fix/main.cpp into multiple files
- Don't warn if the non-zero byte being written is the same as the original byte
- Add a `-O` / `--overwrite` flag to silence all such warnings
Fixes#897
