Add accessibilityAnnouncementEnabled property (default to YES)

We had problems with VoiceOver repeating the announcement of a transient success message we display when the user copies to the clipboard.

By setting the new accessibilityAnnouncementEnabled to NO we are able to ensure the message is only announced once using the following code in our main app:

func presentTransientAlert(message: String) {
    SVProgressHUD.setAccessibilityAnnouncementEnabled(false)
    SVProgressHUD.showSuccess(withStatus: message)
    SVProgressHUD.dismiss(withDelay: 1) {
        SVProgressHUD.setAccessibilityAnnouncementEnabled(true)
    }
}