strokefont.py: Set SFNT "Preferred Family"

At present, this is just the same as the family name.

strokefont.py

@@ -24,6 +24,7 @@ class Stroker(object):
         f.familyname = self.familyname
         f.fontname = self.fontname
         f.fullname = self.fullname
+        f.appendSFNTName('English (US)', 'Preferred Family', self.familyname)
         f.private['StdHW'] = self.nibheight
         f.private['StdVW'] = self.nibwidth
         def adjustblue(y): return y - 100 + self.nibheight