featureWriters: add loadFeatureWritersFromString function
featureWriters: add loadFeatureWritersFromString function

allows to import a feature writer class from a string, useful
for fontmake command line options.

jamesgk/ufo2fdk,googlei18n/ufo2ft,googlefonts/ufo2ft,jamesgk/ufo2ft,moyogo/ufo2ft