Support NO_COLOR

[mstoykov]

There apparently is now somewhat of a "standard" way to disable colors for terminal applications https://no-color.org/, it appears to be fairly popular given the list of things supporting it and fatih/color added support, which is how I found out.

Feature Description

NO_COLOR= k6 run script.js

should not output any color codes

Suggested Solution (optional)

Unfortunately, the above fatih/color update isn't enough as we check ourselves whether we should output color or not. Including --no-color flag which likely just needs to get the env variable source configured ?