Combining quiver and pub_plot

Currently, it seems that one can either plot a quiver plot or a pcolormesh, but not both. It would be nice to be able to do so, perhaps via a command like: 
```
python -c 'import nctoolkit;nctoolkit.open_data("test.nc").quiver_pcolormesh_plot(var="var1",u="u1",v="v1",out="test0.png")'
```