Guidance cards should be stylable

[vincethecoder]

Break out the core functionality in Style to a Stylable protocol. That way, we could extend this card styling in InstructionsCardStyle and provide day style and night style for cards.

Tasks:

• Investigate the best way to expose style properties.

/cc @frederoni @mapbox/navigation-ios

[JThramer]

The Instructions Cards style files should be combined with DayStyle and NightStyle, and we need to integrate the UIAppearance mechanism into the new additions.