Fix: DTS track selection / Added: Batch convert option

Fixed: The script did convert the last DTS track found and not the first one (as it should and as stipulated in the Readme).
Added: When "force" option is set to true, not just the first one but *all* available DTS tracks are converted to AC3

Here is a more readable diff with white spaces (intendation) ignored:
https://github.com/dcthomson/mkvdts2ac3.py/pull/10/files?w=1