Created new Cmap Format

I had a need to access a ttf that had a Format 6 CMAP file, so I created one, and thought I would send it back to you guys. I don't know if Format06.encode works, as I had nothing with which to try it, but I took my best guess. I can verify that the parse_cmap! method does work.

I took a guess as to how to best modify lib/ttfunk/table/cmap/subtable.rb to work with Format 6.