Feature Request: Add mutable extension to NSAttributedString

[thejeff77]

Hey there!

If I have time I might bust this out myself, but if you're in and working on something, it would be really cool to have a simple var extension of NSAttributedString:

var mutable: NSMutableAttributedString

within NSAttributedString

Which would basically return a mutable equivalent of the attributed string.

Best!

[thejeff77]

extension NSAttributedString {
  var mutable: NSMutableAttributedString {
    get {
      return NSMutableAttributedString(attributedString: self)
    }
  }
}