Implement a function to do offset coordinates

[astrofrog]

Turned out to be very easy with a lot of existing astropy.wcs functions:

from astropy import units as u
from astropy.coordinates import SkyCoord

c = SkyCoord(0. * u.deg, 0. * u.deg, frame='galactic')

new_wcs = make_offset_wcs(wcs, c)

import matplotlib.pyplot as plt
fig = plt.figure()
ax = fig.add_axes([0.15, 0.1, 0.8, 0.8], projection=new_wcs)
etc.

[astrofrog]

Need tests and docs...

[astrofrog]

I'm going to try and suggest this for astropy.wcs.utils instead since it's pretty generic.

[astrofrog]

We probably don't need this given SkyOffset frames in Astropy