Handles bit_rate, duration_seconds, and frames for .mkv files

[vnllgrlla]

Added support for .mkv extensions

[drewbitt]

Although this does add support for anything with TAG:DURATION, it's not all mkv. I have a few mkv without the duration tag. For those, the mkv duration is shown without show_streams, but in the xml of show_streams it's "N/A". Will be investigating that for more mkv support.