Even if it might seem that using bat without colors doesn't make too much sense, we should probably respect the NO_COLOR environment variable. There might be situations (bat is used inside another program and the user doesn't even know about it) where it could be useful.

Spec: https://no-color.org/
See similar implementation in fd: https://github.com/sharkdp/fd/pull/551/files (but note: sharkdp/fd#600)