Support formatting ranges of numbers, ordinals, dates, times, and datetimes

[eemeli]

For consideration after the initial release.
See also #125.

We should support formatting (and potentially selection on) both numeric and datetime ranges.

[aphillips]

(chair hat OFF)

This kind of portmanteau issue ("support lots of things") isn't that helpful?

ICU contains a bunch of formatters, including all of those mentioned in the title of this issue, that are not in standard function set. The ICU-TC should almost certainly implement them as namespaced functions for v47 and this WG should consider designs/registration requests for each in the REQUIRED or RECOMMENDED function sets.

The best way to do that, in my estimation, would be individual issues, designs, and specs.