v4.0.0

• Updated sharp dependency.
• Updated yargs dependency.
• Added affine command.
• Added --background, --basename, and --skipBlanks to tile command.
• Removed --density from joinChannel command.
• Renamed --band to --channel (extractChannel command).
• Updated composite command to accept multiple images (#73).
• Updated --kernel option usage in convolve command.
• Updated --offset option usage in linear command.
• Updated matrix positional usage in recomb command.