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

https://user-images.githubusercontent.com/18134219/205460922-8b6a854b-e4e6-4daf-93b8-3e8ed63ca6be.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.

[AlecAivazis]

Seems totally fine to me 👍