Support for "Modified Julian Day" (MJD)

Hi

i'm trying to import a ntpd log to monitor the time skew... but it uses the "Modified Julian Day" (MJD) in the date format and logstash/joda have no way to take this.

i can convert it using date:

```
date -d "57552 days 69728.753 seconds 1858-11-17"
```

I have found this [Page](https://docs.oracle.com/javase/8/docs/api/java/time/temporal/JulianFields.html) that might help

Can you please add support for JD and MJD? even if not needed by ntpd, this are used by astronomers and other scientists, so it might help then import data to elasticsearch.

Thanks
Daniel