Add public initializer to BannerColors

[hkellaway]

To enable clients to do something like this:

class EventLogColors: BannerColorsProtocol {

    private let defaults: BannerColors

    init(defaults: BannerColors = BannerColors()) {
        self.defaults = defaults
    }

    func color(for style: BannerStyle) -> UIColor {
        switch style {
        case .info:
            return .myCustomInfoColor
        case .danger:
            return .myCustomDangerColor
        default:
            return defaults.color(for: style)
        }
    }

}

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