Allow empty sigils as a syntactic sugar for UTC “now”

[am-kantox]

This PR provides syntactic sugar for date/time sigils as shown below

# ~U[]s
DateTime.truncate(DateTime.utc_now(), :second)

# ~T[]m
Time.truncate(Time.utc_now(), :millisecond)

# ~D[]
Date.utc_today()

[josevalim]

Can you please send this up for discussion on the mailing list? I wouldn’t merge it for the empty sigil though. I would prefer ~D[today] or similar, but even then I am not convinced, especially because sigils are often compilation time, so some may expect it to mean the date it was compiled.

Thanks!