Range for add_font_feature

[therahedwig]

So, I had been using the harfbuzz feature string ranges to set feature ranges. But of course, these ranges are per run, as well as being utf32 based.

I'm going to try and see if I'm able to create a range-based function that sets the features over ranges, but doesn't break the runs. Shouldn't be too hard as hb-feature-t is just a struct, but I'm making this issue so I don't forget.