Major Description
If the app needs to render different fields (eg multi-valued vs single-valued) differently it's tricky now.
You can do Passage[0].getMatchTerms()[0].field(), but then that doesn't work if that field hit the empty highlight.
I think we should pass the fieldName to format directly? And then maybe change getMatchTerms() to return BytesRef[] instead (the field name is redundant: they are all the same for all passages passed to format).