Is your feature request related to a problem? Please describe...

Currently, BCalendar shows formated date variant. It is not possible to customize it

Describe the solution you'd like

It should be possible to change format (or at least use BCalendar's context event formats - YMD, Date or Formated).
Some suggestions:

• format="YYYY-MM-DD" - you pass format string (not very ECMA)
• format="YMD" / "Formatted" - same as Context event
• format="someFunc()" - custom formatter

Describe alternatives you've considered

Previously I used HTML native date inputs, then I switched to tempus-dominus (using moment.js). Both options allows custom formatting. It would be nice to have it in bootstrap-vue, too.

Additional context

• In some locales (lt-LT, in my case) long date format is not widely used, therefore harder to read
• Standart ISO YYYY-MM-DD format is shorter and (in some cases) easier to read