Added an open command

Added a new command that allows for opening the source of a python module in the users editor of choice
Ah, OK. But then ```EDITOR``` isn't "the editor" so much as a shell command. I assume that's typical on Unix (I use Windows, where there is no common convention like this).

I still don't like ```shell=True``` though, as you end up needing funky quoting to get the right result. Which you haven't done here... What if ```pkg.location``` contained a space and a ```$``` character? (Hint: The Windows shell doesn't accept single quotes, only double quotes, but it doesn't care about ```$``` characters). Suppose a package installed a file ```bwahaha; rm -r ~```?