Support the microsecond formatting

**Describe the bug**
Currently, day.js is able to parse microseconds, which https://github.com/iamkun/dayjs/pull/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"
```

Start from the format() entry point and compare the microsecond parsing change referenced in pull request #1010. Use the provided timestamp and expected "SSSSSS" output as the acceptance check; done means microsecond formatting preserves and emits all six fractional digits.

Written by the indexing model from the issue text.