Add "N" and "o" format specifiers to idate()

[xPaw]

Moved timelib_isoweek_from_date into case where it's actually needed. And changed the timestamp in tests so day of week difference is clear.

With these formatters, it should cover all the ones date has (except for the ones that return a string, obviously)

[vaske]

build fails

[xPaw]

Travis failed on apt-get install failed, it's not an issue with the PR itself.

[umutphp]

Many thanks for your efforts @xPaw

PS: This is my first review on PHP-SRC repo :)

[xPaw]

Could you reply to #7586 (comment) to say how I should proceed here so this PR doesn't sit forever.

[cmb69]

Thank you! This looks good to me. @derickr, any objections?

[xPaw]

@derickr mind giving this another look?

[derickr]

I have merged this into master now, and I have also updated the documentation: php/doc-en@5e865c8