Support the microsecond formatting

[starwilly]

Describe the bug
Currently, day.js is able to parse microseconds, which #1010 adds.
Can it also support microsecond formatting?

Expected behavior
A clear and concise description of what you expected to happen.

const d = dayjs('2019-03-25T06:41:00.999888');
d.format('YYYY-MM-DD HH:mm:ss SSSSSS')

// "2019-03-25 06:41:00 999888"

[bombillazo]

I second this feature request! I'd like to keep consistency with my DB values as much as possible in my frontend

[SpencerHancock]

Please add this, it makes me sad as my frontend can't provide datetimes with which to query the backend.