Add Dark Mode support and customisation

[AvdLee]

This PR adds support for customization using a TatsiColors protocol. It also adds great support for dark mode by default which might even take away the need to apply custom styling.

• Dark mode support
• Color customization
• Readme updated
• Schemes added to the example project to make it easier to build

Adding customized colors

This can simply be done by using the TatsiConfig.default.colors property which defaults to TatsiDefaultColors(). Simply create your custom struct applying to the TatsiColors protocol:

/// Defines colors that will be applied to the Tatsi elements.
public protocol TatsiColors {
    var background: UIColor { get }
    var secondaryBackground: UIColor { get }
    var tint: UIColor { get }
    var label: UIColor { get }
    var secondaryLabel: UIColor { get }
}

Albums overview

Light	Dark

Recents

Light	Dark

[renssies]

Only 2 small comments, looking good otherwise

[renssies]

I think I would rename this to link as well since you are using the link color instead of the tint color.

[renssies]

A quick description of where each color is used or what it's used for would be great.

[renssies]

Ow and it would also be great if you could enable dark mode in the Example app as well, it just means changing the background colors :)

Correction: Just fixed this for you, it wasn't that much work 😛