Add ability to label the actions office-js takes on the document in the Undo list

Actions performed on a document are added to the application Undo stack. However, the description of the action that you can undo is the same for all actions. Here is a screen shot of Excel after my add-in has altered some elements of an Excel file: 

It would be really useful if we could set the label prior to the next context.sync, e.g.:

    Excel.setUndoLabel("Changed foo into bar");

Of course the label needs to fall back to default after the context.sync() statement.