in Emacs, edit multiple lines at once

It's as simple as this: C-x r t

Some examples are here: http://ergoemacs.org/emacs/emacs_string-rectangle_ascii-art.html

You absolutely need to try installing multiple cursors:

https://github.com/magnars/multiple-cursors.el

It's in marmalade and melpa so just:

M-x package-install multiple-cursors

One of the solutions is using CUA mode. Activate cua mode with M-x cua-mode, select rectangle begin: first press C-Enter then move cursor with standard movement commands to make selection, now pressing enter at any time will cycle cursor through corners of the rectangle enabling you to prepend or append text to the selection.