Added an open command

Added a new command that allows for opening the source of a python module in the users editor of choice
Sorry, mixed up my terminology. My intention is that, if you call 'pip open requests' then the folder for the requests package would be opened.
I just tried running the command again as it is currently and realized that, as you mentioned, it is simply opening the site-packages folder. I will have to revisit the logic so that it opens the appropriate directory.
I am taking my inspiration from the 'bundle open <gemname>' command in the bundler gem that is prevalent in the Ruby world.