Support long options
Use diffusion when -m option is enabled
Add monochrome option
Fix a style issue
Implement monochrome option
Update usage description
Add missing build files
Merge branch 'develop'
Update version
Rebuild
Fix a build error (sixel.h is missing, reported by @ttdoda).
Minor fix
Add --diffusion option
Change the signature of convert_to_sixel
Update help string
Free diffusion argument
Implement --diffusion option
Print stbi_failure_reason when stbi_load failed
Minor fix
Update document