添加AsyncImageAttachment特性支持

Attachment Async Image: (Only supports iOS: UITextView)

// ASAttributedString.AsyncImageAttachment

textView.attributed.text = """

\(.image(url, placeholder: xxxxx))

"""

Custom loader:

ASAttributedString.AsyncImageAttachment.Loader = AsyncImageAttachmentKingfisherLoader.self

Please read the demo AttachmentViewController.swift file for details.