Add color output support

[MatthewMckee4]

Summary

Add colored terminal output to make results easier to scan visually. Line counts, file paths, extensions, and separator lines could use distinct colors. Should respect NO_COLOR environment variable and --color flag.

Example

# Auto-detect (color if terminal, plain if piped)
kount .

# Force color on/off
kount --color always .
kount --color never .

Acceptance criteria

• Line counts displayed in a distinct color
• --color <auto|always|never> flag
• Respects NO_COLOR environment variable
• Auto mode: color when stdout is a terminal, plain when piped
• Use a lightweight crate (e.g., termcolor or anstream)