Option to display fixed width date/time for headlines

[rich-coe]

Add a headline option to always display date/time as %m/%d %H:%M for current year, and %Y/%m/%d for previous years.
Leave as default Today, Yesterday, day names for last week.

[rich-coe]

Add user preference to display date/time in fixed format.
This year displayed as %m/%d %H:%M, otherwise display as %Y/%m/%d.

[bhoefer2015]

Leave as default Today, Yesterday, day names for last week.

That is actually a thing about Liferea that annoys me -- replacing the date column's "%b %d" format with "%a" (or "Today" or "Yesterday") for the past 7 days. I'd rather it always show the name (or number) of the month with the day of the month, at least for the current year.

Although it would be crowded, "%a %b %d" format would also be acceptable.

(
%a locale's abbreviated weekday name (e.g., Sun)
%b locale's abbreviated month name (e.g., Jan)
%d day of month (e.g., 01)
)