Support for desaturate?

Hello

Thanks for the nice plugin. There is one feature that I would like to see be added to it, the ability to desaturate. It is quite simple to add, at its core it would be three additional lines in the 'apply filters' section of the code:

(if (< 0 grayscale)
( gimp-desaturate
drawable)
)

However one have to be careful, if an image/drawable is encountered which cannot be desaturated (non-RGBA layer) his will effectively halt the execution of the script. So before trying to perform the desaturate operation a check has to be made, unfortunately I don't know how though.