support for SOURCE_DATE_EPOCH

[bakkot]

Fixes #390, cc @ljharb.

This lets you do e.g. SOURCE_DATE_EPOCH=$(date -r in.emu '+%s') ecmarkup in.emu out.html to set the date in the rendered specification to the last modified date of in.emu, for reproducible builds.

[ljharb]

@bakkot this helps, but it'd be much easier to make it work on windows as well if it was something ecmarkup could do - either by default, or via a flag.

[bakkot]

@ljharb I am pretty sure you can set environment variables on Windows; what makes Windows special here?

[ljharb]

date isn't available on windows, and i think the syntax to set env vars on windows is different, but maybe cross-env can help.

There's also the issue i'm seeing in the pattern matching repo where despite builds now being reproducible on my machine, CI still finds a difference.

[bakkot]

I am pretty sure there's a way to get the equivalent on Windows, though I don't have a machine in front of me to test. I would very much prefer to stick to the standard option here, if it's at all practical, rather than adding more CLI options.

If you're successfully setting the relevant environment variable and still seeing builds not being reproducible I can look into that; open an issue with a repro?

[ljharb]

Filed #470.