Source cleanliness: share feature file across UFOs

[madig]

This helps keep things in once place, i.e. helps maintenance. Internally, we put shared feature code into src/family.fea and possibly src/family_italic.fea and then just include them in each UFO's feature.fea with include(family.fea). Note that in recent fontmake/ufo2ft versions, the file name is relative to the UFO directory, not the UFO's feature file.

[chrissimpkins]

Thank you!