Output plain template when NO_COLOR environment variable is set

[zimeg]

Summary

This PR checks for a non-empty NO_COLOR environment variable and displays templates without color if one is present, as specified by the references in #447.

Fixes #447

Preview

NO_COLOR.mov

Reviewers

The following steps can be used to inspect these changes:

1. Checkout this branch
2. In a project using survey, append the following to your go.mod, pointing to your local survey source:

replace github.com/AlecAivazis/survey/v2 v2.3.6 => ../../go-survey/survey

3. Run your project with color support by ensuring NO_COLOR is not set:

$ unset NO_COLOR
$ ./your-survey-project

4. Run your project with colors removed from survey outputs:

$ export NO_COLOR=true
$ ./your-survey-project

Notes

• No modifications are made to user-passed prompt values such as Message, Options, Help, and so forth. Removing color from these values is instead left to the caller of this package.

[mislav]

Thank you!

Would you additionally support environment variables CLICOLOR and CLICOLOR_FORCE? https://bixense.com/clicolors/

Reference implementation: https://github.com/cli/go-gh/blob/5f59c9cf4618f1b8074e381209e2f4c0ef9bf25e/pkg/term/env.go#L145-L153

[zimeg]

Happy to change this to match the reference if that's preferred, but wanted to share a way to check if the environment variable is present then check for a != 0 value instead of checking that the variable isn't the empty value.

[mislav]

Thank you! Looks great.

@AlecAivazis How do you feel about having Survey respect environment variables by default?

[AlecAivazis]

Seems totally fine to me 👍