Include Nanoseconds

[tech10]

It would be nice to have nanosecond precision in a human readable duration format.

Given the following code, there are still 784 nanoseconds that could be displayed in an output format. Since output units can be limited to a specific one, I see little reason not to include such a feature, there are already Milliseconds and Microseconds, why not nanoseconds, also?

[ThinkChaos]

Ran into this with tests that were basically doing durafmt.Parse(time.Nanosecond).String().