Add color configuration support: --monochrome flag and NO_COLOR env var

[Copilot]

Motivation and Context

Users had no documented way to disable or customize terminal colors in wego's output. There was no single, unified way to turn off colors across all frontends.

Description

• frontends/colors.go: Added a package-level monochrome variable registered as a single shared --monochrome flag (via init()), and an isMonochrome() helper that returns true when --monochrome is set or the NO_COLOR env var is present
• frontends/ascii-art-table.go: Removed the old --aat-monochrome flag; Render() now calls isMonochrome()
• frontends/emoji.go: Removed the old --emoji-monochrome flag; Render() now calls isMonochrome()
• README.md: New Color Configuration section documenting both approaches

Two ways to disable colors:

# .wegorc — persisted setting for all frontends
monochrome=true

# one-off override via env var
NO_COLOR=1 wego

Steps for Testing

1. Run wego normally — colored output unchanged
2. Run wego --monochrome or wego --frontend emoji --monochrome — ANSI color codes stripped, plain text output
3. Run NO_COLOR=1 wego and NO_COLOR=1 wego --frontend emoji — same result without flags
4. Set monochrome=true in ~/.wegorc and confirm it persists across runs for both frontends

Screenshots

[kordianbruck]

@copilot Please make both frontends just use --monochrome instead of --aat-monochrome and --emoji-monochrome. Also add an option to the wegorc configuration file to set monochrome to be always true.

[Copilot]

@copilot Please make both frontends just use --monochrome instead of --aat-monochrome and --emoji-monochrome. Also add an option to the wegorc configuration file to set monochrome to be always t...

Done in ece35b6. Both frontends now share a single --monochrome flag registered at the package level. It's automatically persisted to .wegorc by ingo, so users can set monochrome=true there to always disable colors. The NO_COLOR env var is still respected as well.