Add --palette-type option
Add new API sixel_output_set_palette_type
Support HLS palette
Merge branch 'release-1.3' into feature-libpng-integration
Cleanup
Fix a typo
Write png data using libpng by sixel2png
Merge branch 'feature-select-palette-type' into release-1.3

Conflicts:
	converters/img2sixel.c
	src/tosixel.c
Merge branch 'feature-use-libpng-writer' into release-1.3