Parse Zulu suffix with %z

[simon04]

Fixes #945.

[pitdicker]

@simon04 Thank you for working on this.

Sorry I never updated #945, but I hope to fix offset parsing in a more complete way with #1082, #1083, and a part three that is ready but does not have an open PR yet.

The idea is to allow more options in the %z specifier to allow colons or not, make offset minutes and seconds optional, and to require padding or not.

The specifier to accept Z would become %#z, and would also be usable for formatting.

In #1083 I was careful to only change offsets in a way that is clearly fixing issues. And in part three 3 the format string is extended in a way that does not really change the meaning of the existing specifiers.

But if we accept this PR now, my plans for extensions to the formatting specifier become messy, or noticeable breaking changes.

[pitdicker]

Closing for the reason mentioned above. Thank you @simon04 for your work here!