Add const methods to ColorSpec

[Property404]

Can now do

const RED: ColorSpec = ColorSpec::new().with_fg(Some(Color::Red));

instead of

let mut red = ColorSpec::new();
let red = red.set_fg(Some(Color::Red));

This is a convenience because:

• A ColorSpec can be defined in a single line
• A ColorSpec can be defined once and used in multiple scopes, without using statics or struct literal syntax

[dimo414]

The .with_ methods in particular would be great additions, thanks!