Support Negative durations.

[Zatte]

When having a LocalDateTime it would beneficial to "remove" a duration instead of only adding it. I can't seem to find an easy way to of turning a positive duration into a negative duration meaning I end up with this pesky workaround.

multiplier := -1
hours, mins, secs, nsec := duration.Units()
negativeDuration := chrono.DurationOf(chrono.Extent(
	time.Hour*time.Duration(hours)*time.Duration(multiplier) +
  	time.Minute*time.Duration(mins)*time.Duration(multiplier) +
	time.Second*time.Duration(secs)*time.Duration(multiplier) +
	time.Nanosecond*time.Duration(nsec)*time.Duration(multiplier)))

Would be neater if this could be done directly on the underlying data representation.
https://github.com/go-chrono/chrono/blob/a79c6fe84b53/duration.go#L12

Adding something like duration.Mul(multiplier int) could probably come in handy.