Add formatters for chrono::time_point<system_clock>

[adamburgess]

Fixes #1819

Makes dates a little easier to format:

auto the_time = std::chrono::system_clock::now();
fmt::print("{}", the_time); // the_time is converted using fmt::localtime, which then uses the normal tm formatter
fmt::print("UTC {}", fmt::gmtime(the_time)); // prints UTC time using a new overload on gmtime/localtime

I wanted to test localtime so I did that using setenv/tzset, though I'm not sure if that's the best way.

C++20 does include a formatter for sys_time -- in #985 it was discussed that printing a time_point was not in the standard library, but it seems with time (hah) it has now been added.

I agree that my contributions are licensed under the {fmt} license, and agree to future changes to the licensing.

[vitaut]

Thanks for the PR. Mostly looks good but the unit tests shouldn't manipulate environment variables. Instead I suggest comparing the output to the result of formatting time_t with system functions.

[adamburgess]

My thoughts were to confirm that passing a time_point would be formatting with the local system time instead of UTC, and that's hard to test without changing the timezone (also: travis etc would be using UTC as well). It's probably up to the stdlib to make sure localtime works ;)

The test now does this: check if time_point formatting is equal to strftime(localtime(to_time_t(now())))

[LarsGullik]

It is a bit bad that the formatters for chrono clocks are not compatible with C++20. In particular that %S in C++20 also prints out subseconds. Going via tm, truncates this to seconds.

[vitaut]

A PR to improve compatibility with C++20 is welcome.