Add support for NO_COLOR environment variable

[EyLuismi]

As issue #143 describes, NO_COLOR is becoming an established environment variable to turn off colour output in environments that may not accept them, such as Log aggregators. More info at: https://no-color.org/

This PR aims to support it by adding an Environment Variable Manager that can be passed down to the Colorizer format and helps with test mocking.

[DABH]

Hey there, thanks for this idea! In principle I have no objection. It seems like this might be a bit over-engineered though - why do we need the env manager? I.e. why not just

this.options.disableColor = process.env.NO_COLOR &&
        process.env.NO_COLOR !== '0' &&
        process.env.NO_COLOR !== 'false';

? I think that's the style we generally use across the winston stack for things (no separate options structs etc.) so that might be a bit more stylistically consistent here (and fewer lines of code). Let me know your thoughts though. Thanks again for your contribution!

[EyLuismi]

Hi! Yes, that was my first approach, but then, the test failed depending on your NO_COLOR ENV var. And using process.env there and in the tests maybe creating unknown side effects seemed like something harder to keep track in the future for the project. I could re-create the PR using that approach if you think is better. Thanks for your time!

[DABH]

Hmm maybe @wbt can take a look and opine sometime on this one