api/resolve: decoding the resp is not needed

We can just use the HTTP Location header.
api: fetch sound metadata

This will give us the necessary metadata to listen to the music.
git: ignore mp3 files

Generated during testing...
api/sound: add DownloadUrl attrib + gen filename

- Nice filenames are handy, same behaviour as SoundScrape, for now.
- Missing DownloadUrl attribute, found it in the API docs, should be higher
  quality if present (Downloadable = true).
api: support downloading tracks

Very rudimentary, no progress bar, but the file I got back was in good
shape!