Add --color=auto|always|never to control ANSI output (#81)

[raphaelsty]

Closes #81.

Problem

colgrep emits color through two independent channels:

1. the colored crate (.cyan() file headers, .dimmed() line numbers), and
2. raw ANSI escapes from syntect syntax highlighting in display.rs.

colored honors NO_COLOR, but syntect doesn't — so output always contained escape sequences regardless of NO_COLOR/CLICOLOR/CLICOLOR_FORCE/TERM. As @divaltor noted, that pollutes context for agents/plugins that consume the output directly rather than through a terminal.

Change

A small color module resolves a single decision from --color and applies it to both channels:

• sets the colored crate override (so every .cyan()/.dimmed() obeys), and
• records the result for the syntect path to consult via colorize_enabled().

display.rs now prints plain, line-numbered text (no syntect, no \x1b[0m) when color is off.

--color auto     # default: color only on a TTY, honoring NO_COLOR / CLICOLOR / CLICOLOR_FORCE
--color always   # force color even when piped
--color never    # plain text, zero ANSI escapes

The flag is global, so it works on every subcommand (colgrep --color never ..., colgrep search --color never ..., etc.). Env handling in auto: CLICOLOR_FORCE (non-empty, non-0) forces on; else NO_COLOR (non-empty) disables; else CLICOLOR=0 disables; else colorize only on a TTY. An explicit --color always/never always wins.

A nice side effect: because auto now respects the TTY, piped output is plain by default too — which is the behavior @divaltor originally expected from NO_COLOR.

Testing

Unit tests for the env/auto-detection and the global toggle. End-to-end (verbose -c path, which is what triggers syntax highlighting), counting actual ESC (0x1b) bytes in captured output:

Invocation	ESC bytes
--color never	0
default auto, piped (non-TTY)	0
NO_COLOR=1 (auto)	0
user's exact repro + --color never	0
--color always	1047
CLICOLOR_FORCE=1 (auto, piped)	>0
NO_COLOR=1 --color always	>0 (explicit wins)

Plain output keeps line numbers and content intact; cargo fmt/clippy/full colgrep test suite (541 lib + 55 bin) green.

Note on the follow-up ask

The issue also floated "might add an option to disable line numbers." I kept this PR scoped to color since that's the titled request and what the maintainer comment committed to; happy to add --no-line-numbers in a follow-up if wanted.