feat: Introduce support for DateTime skeleton

This bumps the version of `intl-messageformat`, thus introducing a
subset of
[DateTime Skeleton](/~https://github.com/formatjs/formatjs/tree/master/packages/intl-messageformat-parser#supported-datetime-skeleton)
to react-intl.
The skeleton syntax per ICU is `today is {ts, time, ::yyyyMMdd}`.
Further documentation can be read at
http://userguide.icu-project.org/formatparse/datetime.