Error out when a flag that expects a value is given without one

Flags like --logo, --speed, --frames, --shading-chars, --shading-mode,
--height, --size, and --depth were only matched when an argument
followed, so calling them with no value silently fell through to the
unknown-option branch with a confusing message. Check for the flag
name first and report a clear "requires an argument" error instead.