Change Timestamp::fmt to format year as YYYY

[KieranKaelin]

Hey there, thanks for the great crate!

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The Timestamp::fmt trait does not pad the year to four digits, resulting in dates such as year 2 being formatted as 2-01-01 00:00:00.

[KieranKaelin]

Hi @kubo,
Just wanted to kindly follow up on this PR. Let me know if there's anything else I can do 😄
Thanks for your time!

[kubo]

@KieranKaelin
Sorry for too late merge.
It will be included in 0.7.0, which supports ISO 8601 extended format when alternative form "{:#}" is used.