Improved KernFeatureWriter for LTR/RTL kerning

This implements what was discussed here unified-font-object/ufo-spec#16 (comment)

The result's have been used but are not yet approved by anyone :-)
This is the output file: https://github.com/Tarobish/Jomhuria/blob/master/sources/kerning.fea

@behdad wrote:

> * Generate both an LTR kern lookup and an RTL one; including all kerning pairs in both.
  * Reference the RTL lookup from script systems that are RTL, and the LTR one for others.
>
>That's the crudest heuristic, but it works. From there on, you can add more complexity to achieve smaller kern tables. But none of these really belongs into the UFO per se. For example, a huge improvement:
>
> * Associate each glyph to a Unicode character,
  * Exclude from RTL kern table all glyphs associated with Unicode characters that have Bidi_Type=L,
  * Exclude from LTR kern table all glyphs associated with Unicode characters that have Bidi_Type=R or Bidi_Type=AL.