ICU has the following options:

1. "halfUp" (default)
2. "halfEven"
3. "halfDown"
4. "ceiling"
5. "floor"
6. "up"
7. "down"

Chart: http://userguide.icu-project.org/formatparse/numbers/rounding-modes

Do we want to include all those in Intl.NumberFormat? How would we pick a subset?

@waldemarhorwat @littledan