Add a 'quicklook' viewer for slices

Add an aplpy-based quicklook tool.  E.g.:

```
cube = SpectralCube.read('cube.fits')
cube[50,:,:].quicklook()
cube[25:50,:,:].sum(axis=0).quicklook()
cube.spectral_slab(0*u.km/u.s, 10*u.km/u.s).quicklook()
```