improve podPress file handling

- fix m4a mime type
- respect "mediaWebPath" setting
enhance autoenclosure-detection support

if there are multiple enclosures, we now generate one player with
multiple sources rather than one player per enclosure
displays chapter duration without leading zeros. fixes https://github.com/podlove/podlove-web-player/issues/33
simplifies and fixes padding of chapter start time code
Merge pull request #36 from Kambfhase/Kapitelmarker

displays chapter duration without leading zeros.