feature request for date: add formatting option for milliseconds

[Phil Oertel]

I'm just a user, using date to do some script profiling. Seconds are too
coarse, and nanos are too hard to read. I'd like to have millis (and sure,
maybe micros while we're at it).

I propose a new formatting option for millis past the nearest second (as
opposed to millis since epoch. Same semantics as the nano option.) And
sure, one for micro too.

You could do this already by truncating digits from nanos, but it'd be a
lot more convenient if date had an option, and hopefully not too hard to
add.

I'd be open to implementing the feature myself, but I'd like to get input
on the idea first.

Thanks!
Phil