Add colorization for headers in patch preview

[davidbtadokoro]

Description:

Recently, we've added styling to patch previewing with the external renderers bat, delta, and diff-so-fancy. However, the message's headers (Subject, From, etc.), which are constant in all patches, are still flat white (see screenshot below). As these headers are well structured, we could add a rendering for colorization.

Motivation:

This shouldn't be hard, and the colorization is sure enough welcome.

[OJarrisonn]

I've just opened a PR for this