Add a way to add font features to a PangoFontDescription. The new
API in this commit are
pango_font_description_get_features
pango_font_description_set_features
pango_font_description_set_features_static
PANGO_FONT_MASK_FEATURES

When serializing to a string, we append font features like this:

@@feature1=1,feature2=2,...