New Countdown extension

[GiovanniSalmeri]

Yet another small extension: https://github.com/GiovanniSalmeri/yellow-countdown

The extension Countdown creates, well, a countdown timer. Some noteworthy features: it has a "textual mode" when used in-line; it adapts automatically the precision when the time is running out; it fetches the time from the server (not from the computer of the user). The code generated is given the role of timer and should behave well with screen readers. As in the spirit of Yellow, the code is very light. Enjoy!