kernFeatureWriter – respect public.skipExportGlyphs

[frankrolf]

Currently, kfw will just export groups/kerning for all glyphs that are somehow involved in a kerning pair.
The problem here is that some glyphs might be excluded via the GlyphOrderAndAliasDB, resulting in an invalid kern feature.

It would be smart to leverage the UFO lib key public.skipExportGlyphs for skipping those glyphs.

h/t @arialcrime