title: central longitude for NorthPolarStereo
body: <p>I'd like to plot a polar stereographic plot of the Northern Hemisphere with 180 at the bottom of the plot so I can emphasize the Pacific region. I'm using the latest cartopy from git, and can make a polar stereographic plot no problem, but I can't work out how to change which longitude is at the bottom of the plot. I tried setting the longitude extent to [-180, 180] but this doesn't help, and the NorthPolarStereo() doesn't accept any keyword arguments like central_longitude. Is this possible currently?</p>