request ability to control colored output

May we have a way to turn off the colored output from git-spice please? Or change which colors it uses?

git-spice output is very hard to read using the [solarized dark](https://ethanschoonover.com/solarized/) color scheme because Solarized sets the ANSI bright codes to grey scale.

The last line "PR ## was merged but local SHA does not match" is specifically difficult to read:

I tried `git config --global color.ui false` but it had no obvious effect.