Include pip config --editor option which allows execution of arbitrar…

[raj3shp]

• pip config --editor allows specifying an editor for pip config changes.
• User can pass any filepath as editor, this allows spawning a shell or escalate to root if user has sudo capability to run pip.

[raj3shp]

Hello @cyrus-and / @epinna , can I please get a review on this? Wondering if this project is accepting new contributions. I have couple of more to add.