Merge branch 'feature-load-grayscaled-input' into develop
Merge branch 'develop' into builtin-vt340-palette
Style improvement
Add new built-in palette definition, VT340 mono and VT340 color
Add new argument definitions of -b option, vt340mono and vt340color
Merge branch 'builtin-vt340-palette' into develop
Expand 8bpp grayscale input into 24bit colors if it is needed
Make palette from grayscaled PNG with -m option
Add some tests of loading grayscaled PNG
Minor change of mehod of creating histogram
Merge branch 'develop' into feature-load-grayscaled-input