Fixes #155

Adds support for the FORCE_COLOR environment variable. When FORCE_COLOR is set to any value other than "0", colors are enabled regardless of TTY detection. NO_COLOR takes precedence over FORCE_COLOR.

This is useful for CI systems like GitHub Actions and GitLab CI that support ANSI colors but aren't detected as TTYs. The FORCE_COLOR convention has been standardized at https://force-color.org and is already widely supported in the Node.js ecosystem (chalk, etc.) and increasingly in other languages.

Changes

• color.go: Added forceColorIsSet() helper and integrated it into the NoColor global initialization and New() constructor
• color_test.go: Added tests for forceColorIsSet(), color output with FORCE_COLOR, and NO_COLOR precedence over FORCE_COLOR
• README.md: Documented FORCE_COLOR support and updated the GitHub Actions section

Testing

$ go test ./... -v
--- PASS: Test_forceColorIsSet (0.00s)
    --- PASS: Test_forceColorIsSet/default (0.00s)
    --- PASS: Test_forceColorIsSet/FORCE_COLOR=1 (0.00s)
    --- PASS: Test_forceColorIsSet/FORCE_COLOR=0 (0.00s)
    --- PASS: Test_forceColorIsSet/FORCE_COLOR=true (0.00s)
    --- PASS: Test_forceColorIsSet/FORCE_COLOR= (0.00s)
--- PASS: TestForceColor (0.00s)
--- PASS: TestForceColorOverriddenByNoColor (0.00s)
PASS
ok  	github.com/fatih/color	0.239s