use `<time>` tag for datetime

[scarf005]

Adding <time> tag helps with accessibility, such as screen readers.

one thing i'm worried about is that it'll cause breaking changes:

• add_spans should be renamed to wrap_tags to not be misreading (as span != time)
• css rules targetting old spans would be broken (span.git-revision-date-localized-plugin)

I'm willing to open a PR if these changes would be acceptable.

[timvink]

Sounds good!

add_spans should be renamed to wrap_tags to not be misreading (as span != time)

Let's not introduce any potentially breaking changes. Just add a comment in the code if you think it's unclear for any (future) developers. The code is anything but clean tbh, but it's stable and a small package, so no need to refactor.