Added an open command

Added a new command that allows for opening the source of a python module in the users editor of choice
Yea, ``VISUAL`` and ``EDITOR`` are \*nix conventions for a shell command that opens up the users desired editor. As far as I know the convention is YOLO don't open up untrusted input, but I could be wrong! One option would be to copy any files to a temp file location which we know has a safe name for editing, and then move it back when the editor closes. This could also play into the "reset" functionality since we'll control the life cycle of the changes in that case.