v0.6.5

Added

• Countdown. Time to a moment or since one, counting either way, with antd's
  format tokens — Y M D H m s S, padded to the width of the run, with square
  brackets kept as written. A unit left out of the format rolls into the next
  one down, so HH:mm:ss reads 26:00:00 where D[d] HH:mm:ss reads
  1d 02:00:00.

  Named Countdown rather than Timer: dart:async already has one, and it
  is needed in the very file that shows this widget.

  A countdown rounds up to the smallest unit its format asks for — three and a
  half seconds left reads 00:04, since formatting the remainder as it stands
  opens a fresh countdown one short of its own length. It wakes only when the
  drawn text is due to change, and measures against the wall clock, so it
  stays right across a suspended app.