Dark Mode Support

[rlaurb]

As of 9.2.1, Marky-Mark does not support dark mode. I have made this change in my local copy, but suggest that adding this to Marky-Mark on an official basis would be both simple and valuable.

The approach I used was to change all references to specific colors in the Marky-Mark code to new custom colors. For example, I replaced all references to UIColor.black to UIColor.mmText, where mmText is defined below:

extension UIColor {
	static let mmLigthTranslucentGray : UIColor = {
		if #available(iOS 13, *) {
			return UIColor.systemGray4.withAlphaComponent(0.25)
		} else {
			return UIColor.lightGray.withAlphaComponent(0.25)
		}
	}()
	
	static let mmGray : UIColor = {
		if #available(iOS 13, *) {
			return UIColor.systemGray
		} else {
			return UIColor.gray
		}
	}()
	
	static let mmText : UIColor = {
		if #available(iOS 13, *) {
			return UIColor.label
		} else {
			return .black
		}
	}()
	
	static let mmBackground : UIColor = {
		if #available(iOS 13, *) {
			return UIColor.secondarySystemBackground
		} else {
			return UIColor.gray
		}
	}()
	
	static let mmGrayText : UIColor = {
		if #available(iOS 13, *) {
			return UIColor.secondaryLabel
		} else {
			return UIColor.gray
		}
	}()
}