ISO 8601 year (%G and %g)

[davinov]

Following the example of #37 and the documentation of strftime, I suggest adding the %G and %g specifiers.

In time-series with a weekly granularity spanning on more than one year, the format Week %V %Y seems appropriate.
However, for the end of 2018, it displays the following: Week 52 2018, Week 01 2018.
This is because the first day of the first ISO week of 2019 is December 31st 2018, hence is year is 2018.
Instead, we would like to display the year that correspond to the ISO week with Week %V %G: Week 52 2018, Week 01 2019.

The parsing operation doesn't need any work according to tests: %G and %g are just aliases for %Y and %y.

[davinov]

Thanks @BrianMitchL for your work on #37 that makes this PR easy.
I just noticed that you started with the same idea on #49 :s

[Fil]

Do you want to rebase it? Thank you

[davinov]

Do you want to rebase it? Thank you

It's been a while :D I will do so

[davinov]

@Fil Rebased :)

[Fil]

Testing, I was suprised that there are sometimes 53 weeks!
"2021-01-03 = Sun week 53 2020"
but I cross-checked with http://blog.stevenlevithan.com/archives/date-time-format and with https://www.epochconverter.com/weeknumbers, and it seems to be true :)