office文件的iOS-UTI支持

系统声明的统一类型标识符
https://developer.apple.com/library/content/documentation/Miscellaneous/Reference/UTIRef/Articles/System-DeclaredUniformTypeIdentifiers.html
https://developer.apple.com/library/content/documentation/FileManagement/Conceptual/understanding_utis/understand_utis_declare/understand_utis_declare.html
http://paulsolt.com/blog/2012/02/creating-nsdocument-using-folder-bundles-and-uti-identifiers
http://sunxiang0918.cn/2015/11/27/%E4%B8%BA%E8%87%AA%E5%B7%B1%E7%9A%84APP%E5%A2%9E%E5%8A%A0OpenIn%E5%8A%9F%E8%83%BD/
Cocoa Programming L22 - Saving Documents
https://www.youtube.com/watch?v=yfiOGQmUVjE