Request: Allow saving of previewed file

It would be quite convenient to be able to save the preview file as it is, with the appropriate file extension. Just as any other pane/tab, say with `Ctrl-S`.

For example, if previewing compiled Coffeescript, the tab could show the coffee filename, but with a `.js` extension, and it is treated like a new file that can be saved with `Ctrl-S` if it's in focus.