Respect TERM=dumb more consistently

[ylh]

This is a huge quality-of-life improvement for my niche use case:

[edolstra]

Looks good, but maybe instead of TERM=dumb we should respect NO_COLOR instead or additionally (see https://no-color.org/).

[ylh]

First time hearing of that convention, but by the looks of it, checking for both seems like the most sensible option. I've tacked on a commit accordingly.

[edolstra]

Thanks!