Add program datetime from system or stream timestamp

This fixes #373, adding the tag `EXT-X-PROGRAM-DATE-TIME` to HLS playlists.

The new directive is `hls_datetime` and can be set to:
* `none`: (default) tag is not added to playlist.
* `system`: uses system time.
* `timestamp`: uses RTMP timestamp, but since timestamps in RTMP are relative to an unspecified epoch, it is not precise, just consistent.