Add  Skycoord.to_pixels_wcs

This is a fairly trivial add on to #2885 that adds a convinence method to go from SkyCoord to pixel coordinates.

Not that there is *not* an inverse method, because it would make most sense as a classmethod, and `astropy.wcs.utils.pixel_to_skycoord` only uses the `SkyCoord` object explicitly.  It wouldn't be too hard to change that, though... What do you think, @astrofrog?
So to be clear, @embray, it could be moved to module-level if there's a specific reason, but it seems more readable to me this way (which is admittedly subjective).  Feel free to merge this if you're fine with that, or if there's some significant reason or guidline/whatever I'm missing on this, I can move it to the module-level and then merge.