join for NSAttributedString

NSAttributedStringExtension.swift

extension CollectionType where Self.Generator.Element : NSAttributedString {
    func join(separator: String? = nil) -> NSAttributedString {
        let mutableAttributedString = NSMutableAttributedString()
        self.forEach {
            if let separator = separator where self.first != $0 {
                mutableAttributedString.appendAttributedString(NSAttributedString(string: separator))
            }
            mutableAttributedString.appendAttributedString($0)
        }
        
        return mutableAttributedString
    }
}