ColorCodes: eliminate NBS_ISCC sub dictionaries from lookups.

By their own documentation, the NBS submodules contain idiosyncratic
color names which are not well-suited to a general purpose color code
search.

In particular, NBS_ISCC::M contains a color called 'email' which caused
a confusing result for 'HTML email'.

The main NBS colors are included because they seem reasonable enough as
color names.

This fixes #490 and possibly partially addresses #454 and #442, I guess.
Personally, I'd definitely add tests based on the ones that I know I've excluded; that way if someone refactors the implementation we can guarantee they've not introduced any regressions :)