Add Day of Year Support

[ZachDischner]

It would be excellent to be able to include 3 digit day-of-year support for date formats.

For example, using datetime

In [14]: datetime.datetime.strptime("2016-360","%Y-%j")
Out[14]: datetime.datetime(2016, 12, 25, 0, 0)

The equivalent with arrow could be

arrow.get("2016-360")

For background, most of the datetimes I work with for spacecraft telemetry timestamping utilizes the day-of-year format since it is is pretty locale agnostic (2016/12/11 vs 2016/11/12).

[Zamubafoo]

While an implementation of the 'j' token would be a nice feature, you can easily extract the date into arrow without much trouble:

dayOfYear=arrow.Arrow.strptime("2016-360","%Y-%j")

This gets you to the correct time inside an Arrow object which is easy to work with, then you can export it via:

dayOfYear.strftime("%Y-%j")

It's nothing amazing, but it definitely beats datetime and the bunch.