Added an open command

Added a new command that allows for opening the source of a python module in the users editor of choice
The issue I was running into is if I have the EDITOR variable set to something like 'emacs -nw', then Popen threw an error that 'emacs -nw' wasn't found. Setting shell=True fixed that, as the variable was being interpreted by the shell. It also allowed for the case when the editor is vim because otherwise the vim process would terminate before being displayed in the terminal.