Implement formatUnit

As discussed in #205

The test I wrote is not running, I don't know where should I add it.
Also I guess we need to add tests for all units not just the duration related ones.
Note to cldr: month and minute have the same short format `m`. This is bad :)

Also I think we need to support Date, 2 Date and auto-unit-detect versions of those methods, just like `formatDuration`, at least in date context.