ke/map_ke are not official .mtl specs, but seem to be rather common extension.
map_ke was added three years ago to exporter only, this commit finishes the work
by adding ke/map_ke to importer, and ke to exporter.

NOTES:
* mtl ke is a color, while in Blender emit is only a factor, using diffuse color.
  this implies there is some loss of data during import here.
* I do not have the slightest idea about the range for ke values, so for now assuming
  a direct matching of Blender's emit value.