Request: indexed png colormap size control

It would be helpful to have control of the size of the colormap that  
is created when using the png8 format. Currently using -colors forces  
the minimum size possible. I would suggest this syntax, where X is  
the minimum size and Y is the maximum.

-palette-bits X:Y
	or
-colormap-size X:Y


The reason that I am asking is that I am using convert to create  
images for mobile devices (cell phones) and the image decoders are  
poorly written and buggy. Many devices only decode png8 files with 8- 
bit colormaps. This makes the -colors option unusable on these devices.

Thanks for the great tool,
-Mikel