Storing your files in Dropbox is the exact solution isn’t it? They auto-sync. Or do you mean that you don’t want the files to be stored locally at all?
Or do you mean that the editor keeps your account so that whenever you install the editor and log in, it finds all your files?
I don’t know exactly the workflow you want. Box.com also has a feature called “Box Edit”. If your file is not stored locally, and you’re browsing your cloud files online, it will let you pick a file to edit and Box Edit will automatically sync and open the file locally, then when you save it, it syncs back to the cloud. You would not even have to have Box installed or syncing your folders to do this, so you wouldn’t need local storage in that case.