Fix error when output filename is an url

[JOJ0]

• Fixes Alternate metadata sources that add a URL as mb_trackid cause xtractor JSON write to fail #16
• mb_trackid contains an URL populated by some metadata source plugins (eg.
  bandcamp)
• use the md5 filename generation lines (that were already there for cases
  where mb_trackid is not present at all) in case mb_trackid contains a slash
  (/) character
• could be advanced later if other unwanted characters populated by source
  plugins are found in mb_trackid but should be a good enough for now fix.

[JOJ0]

Hi @adamjakab I'll try to submit some fixes that were reported as issues as well as add som things I think are useful with my own current "experiments" on getting the plugin running.

How should we proceed with dev in general? I know you are a busy man but quick reviews/approvals from you would help me a lot. This one should be a no-brainer though, since @doctorfree uses it in his latest musicplayerplus release already. I think it's a perfectly fine fix.

I'll submit some more fixes and addition in the next hours/days and I'll try to keep them tiny so they are quick to review.