[core] add a date property to the countdown slide template

[peterpeterparker]

A new countdown template has been recently added (#45). Currently it's possible to using using properties hours, minutes and seconds.

I think it would be cool to extend it in order to be able to pass a date a property ("My presentation start Friday 30th May 2019 at 19:00"). Using this date, the countdown should be displayed (max value: 59:59:00).

Also when the date is over either we display 00:00:00 or maybe we could use a slot to let the user decide what he want to display ("My presentation starts not" or "My presentation started Friday...").

[peterpeterparker]

Regarding my last comment in the description, for now on at least, as I didn't found a cool way to solve it, the component will still display 00:00:00 when time is over even if date is used. If anyone has got a cool idea of to solve this with a nicer UX or design, ping me