Parse mkv files for SegmentUID and Duration without MKVToolNix

- Eliminates the dependency on MKVToolNix if audio trimming is not required. Not that I really expect someone to work with mkv files and not use it.
- About 700x faster parsing, very noticeable on directories with a large number 
  of mkv files.

This is something I did just for fun, the speed improvement was a nice surprise. The Matroska files are not parsed properly, instead the needed elements are just directly searched for in the file. It works correctly on every mkv file I tried, from different muxers, but just in case there's also a new option on the first lines of vfr.py for using mkvinfo again.