Added an open command

Added a new command that allows for opening the source of a python module in the users editor of choice
* Ok, that's a good reason for ``shell=True``.
* So my question really is, what is "the location for foomodule". If the "cffi" distribution has a single .py file should it open up that single file? If it has a single package should it open up that single package's folder? As is most of the time this is going to open the ``site-packages`` folder which will have a whole bunch of files in it unrelated to whatever you passed in to this command. I'm not saying that's the wrong answer, just exploring if it's the right answer or not.