Added an open command

Added a new command that allows for opening the source of a python module in the users editor of choice
First of all, you don't need ```shell=True``` to search ```PATH``` (at least, I know you don't on Windows).

Secondly, editors don't open directories, so I'm confused. I know some editors can be given a directory name and they open some form of directory browser, but you can't assume a user will have set their default "editor" to something that did that.