Add timezone support to clock module (closes #223)

[mjec]

Please forgive my C++ newbiness!

Notably I had to do the subprojects thing to get around HowardHinnant/date#398, and I don't love that I had to write my own formatter in clock.cpp, but I think this should work.

Feedback, and push back, very welcome. Let me know how to improve the code and I will!

[layus]

Here are the issues I discovered:

1. date is not trivial to package, at least with nix. It has incomplete cmake support (work is ongoing apparently) and no meson support. mesonbuild wrapper cannot be used as it downloads stuff on the fly. But that is a concern for package maintainers.

2. My TZ dir is /etc/zoneinfo as specified by my $TZ_DIR var. apparetly date does not look at TZ_DIR. this sould be upstreamed, but in the meantime this can be a breaking change.

3. The current setup does not follow LC_TIME (fr_BE.UTF-8 here). I do not know if it follows any locale, or just default to a C/en_US locale. This is a breaking change.

4. With several clocks, it may be useful to use a different locale per clock. But this can be left out for later.

[layus]

From what I see here there is a version of date::format that takes the locale as its first argument. We should use that with std::locale("") or find a way to default to the right locale.

[mjec]

With several clocks, it may be useful to use a different locale per clock. But this can be left out for later.

I like this idea, but after noodling with it for a bit my C++ just isn't up to the task :(.

My thinking was having a new struct containing a localtime and locale that gets passed to the fmt::format, and then the locale can be instead of std::locale in formatter::format. However I wasn't able to figure out a way to deal with date::zoned_time being a template type (I think that's the phrase for it?).

[layus]

@Alexays I think we have reached the full set of desirable functionalities. Could you have a second look to check C++ style ?

[Alexays]

Why not just use !config_["timezone"].isString() and removing fixed_time_zone_ var?

[layus]

To avoid going through the json parser each time. May not be worth it.

[Alexays]

The json is already parsed, but yeah, never mind.

[Alexays]

LGTM Thanks guys ! ❤️

[mjec]

@Alexays Have updated the patch URL to be github as well, and rewrote history a little bit to use my personal (rather than work) email on the commits.

Let me know if you'd like me to rewrite the history/squash commits at all.

[Alexays]

Thanks guys, great work 👍

[dancju]

Will wiki page update?

[Alexays]

Man page is updated, but the wiki still need to be updated, @mjec can you give a shot?

[mjec]

Just updated the wiki @Alexays

[mjec]

Yes @Nerddan, with a config like:

{
    "modules-right": ["clock#nyc", "clock#sydney"],
    "clock#nyc": {
        "timezone": "America/New_York"
    },
    "clock#sydney": {
        "timezone": "Australia/Sydney"
    }
}

[dancju]

When will this be packed into the next release?

[Alexays]

@Nerddan By the end of the week