Adds Designable UILabel `Tracking` and `Line Height` for UILabel.

DesignableLabel.swift

@IBDesignable
class DesignableLabel: UILabel {
    
    // Tracking
    @IBInspectable var tracking: CGFloat = 0.0 {
        didSet {
            if attributedText?.length == nil { return }
            
            let attrStr = NSMutableAttributedString(attributedString: attributedText!)
            let range = NSMakeRange(0, attributedText!.length)
            attrStr.addAttributes([.kern: tracking], range: range)
            attributedText = attrStr
        }
    }

    
    // Line Height
    @IBInspectable var lineHeight: CGFloat = 20 {
        didSet {
            let paragraphStyle = NSMutableParagraphStyle()
            paragraphStyle.minimumLineHeight = lineHeight
            paragraphStyle.maximumLineHeight = lineHeight
            paragraphStyle.alignment = self.textAlignment
            
            // Take into account tracking (if already set)
            var attrString: NSMutableAttributedString = NSMutableAttributedString(string: text!)
            if let existingAttributedText = attributedText {
                attrString = existingAttributedText.makeStringWith(text: text!)
            }
            
            attrString.addAttribute(NSAttributedStringKey.font, value: font, range: NSRange(location: 0, length: attrString.length))
            attrString.addAttribute(NSAttributedStringKey.paragraphStyle, value: paragraphStyle, range: NSRange(location: 0, length: attrString.length))
            attributedText = attrString
        }
    }
}