Added deckgo-slide-countdown slide web core component

[gauravmahto]

This commit adds deckgo-slide-countdown web core slide component.

Please see #45

Usages:

<deckgo-slide-countdown hours="0" minutes="0" seconds="5">
  <h1 slot="title">My presentation will start in</h1>
</deckgo-slide-countdown>

Exposed public method on the HTML element:

start() - To start the countdown.
stop() - To stop the countdown.

Exposed CSS custom properties:

--countdown-base-size : Initial component base size. 
--clock-background-color : Clock's background color
--clock-foreground-color : Clock's digits' color.