Wrap post times in html time tag

Post dates and times should be wrapped in HTML5 time tags. It's simply more semantically correct, and better parseable for search engines too.

https://developer.mozilla.org/en-US/docs/Web/HTML/Element/time

https://stackoverflow.com/questions/6852866/what-s-the-point-of-using-the-html5-time-tag

It would also make it easier to implement some JS code to customize date formatting client-side, like using moment.js through an extension.