Suppress color codes when stdout is not a terminal.

[showell]

Discussion here:

https://chat.zulip.org/#narrow/stream/43-automated-testing/topic/lint.20color.20codes/near/953792

Hello @showell , I am really interested in solving this issue..
Can you please guide me in how to get started with this. I am new to open source but really eager to contribute to this linter project.

[andersk]

We should also check for the quasi-standard NO_COLOR variable.