colorbar is missing documentation for "-I" and lacks a "shading" alias

[MarkWieczorek]

In practice, whenever an image is plotted using shading ("grdimage -I...") it is common to use the "-I" option for the colorbar as well. At present there is no documentation of the colorbar "-I" option, but it turns out that you can get this to work anyways. Two calling options are

fig.colorbar(I='')

or

fig.colorbar(I=True)

For simplicity, I would probably document the second option.

Furthermore, for consistency with grdimage the colobar function should have an alias called shading so that we don't have to use the cryptic gmt short commands.

[seisman]

Closed by #752.