feat(dateFilter): Add option to display date as am/pm

This adds a new option in order to choose whether to display a date as
AM/PM or am/pm.

Note: This contains a breaking change.  `a` now will display am/pm,
whereas the old behavior is specified by `A` for AM/PM.  The default
values for the shorthand date displays are also changed here.

Revert to old display for shorthand formats