Description of the desired feature

The main use of one-liners is to quickly make a plot of a data set, such as a grid. It is not meant for production of final figures (unless this very simple plot is good enough). However, plotting a grid via grdimage with a master CPT or default will scale the colors to the range of the grid, hence it is unclear what the colors represent unless one

1. Creates a know-range CPT via makecpt/grd2cpt and use that in grdimage
2. Abandons the one-liner in order to place a colorbar in a modern mode script.

I suggest that when a one-liner involves a current CPT (either implicitly or explicitly) we add a

gmt colorbar -DJBC -B
at the end of the one-liner call so that it is possible to make sense of the plot. Anyone object to this backwards-incompatible upgrade/"bug fix"? @GenericMappingTools/core ?