Move selected cells

This is a quick (and dirty) extension - move up or down several selected cells. Moving cells or series of cells via simple keystrokes can be super useful.

It is a bit dirty because it would be better to act on DOM elements and write a correct move_cells() function.

Cautionary note: It is very probable that such functionality will be available shortly in the official Jupyter notebook. But in the meantime, it could be useful to some people.

Keyboard shortcuts: Alt-up and Alt-down (works also with single cells!)

Cell selection: Cells can be selected using the rubberband (required extension) or via Shift-J and Shift-K