support exporting vertical kerning to UFO

[anthrotype]

There's a kerningVertical dict in .glyphs sources, which glyphsLib can parse and serialize from/to .glyphs files. We need to find a way to export that to the UFO in a way that fontmake can then use with ufo2ft (googlefonts/ufo2ft#668) to compile vkrn feature.

[schriftgestalt]

I see two options:

• Write it to verticalKerning.plist and add support for it in fontmake/ufo2ft.
• put it in a private key in lib.plist and add support for that in fontmake/ufo2ft.

I’ll add the private key for now just to be able to round trip.

And we also need to find a place to put the vertical kerning classes. When we put them in groups.plist we need to make sure that there are no collisions with the horizontal groups. In Glyphs it might be that leftKern, rightKern, topKern and bottomKern are all the same (e.g. "A")