Define format for dt-duration

[dshanske]

http://microformats.org/wiki/h-event defined dt-duration as a stable property defined as the duration of the event. However, it does not show an example nor define a format.

hCalendar and iCal defined its duration property as an ISO8601 duration.

So, logically, the format should look something like: <time class="dt-duration" datetime="PT14M35S">14 minutes and 35 seconds</time>

@snarfed argued(snarfed/granary#169 (comment)), however, that ISO8601 durations are rare in the wild and seconds was much more universal. This was in a media file context, however.

[dshanske]

Related microformats/h-entry#21

[dshanske]

Related https://indieweb.org/duration

[dshanske]

Oops, I missed this is already #5