Made MkvtoMp4 class much more dynamic

Added the following options which were handled statically before:
- Enable/disable deleting of original file (previously was always
deleted)
- Change output extensions (mp4 was only option previously, now supports
mp4/m4v)
- Added option to specify alternative output directory (previously just
used same directory as input file)
-Restructered script so new file extensions for both input and output
can be added easily. Much cleaner approach.
Increased functionality will be implemented into the tvdb_mp4.ini file
to allow more flexibility of the script in the future.