Matroska container header meta data parsing

[GenericGuy]

From what I've gathere online, Matroska (mkv) files don't provide the same interface as MP4 files for reading metadata. Duration and creation date are not stored in the stream section, but in the container section.

Mainly 3 additions:

• Duration is read from the format section only if it's not set already (haven't found too much information on this, but based on my personal files the stream metadata seems to be more accurate for the duration)
• Bit rate from the format section overwrites previously found bit rates (since the stream bit rate really only accounts for video, it makes more sense to use the average overall bit rate)
• Creation date also overwrites previous creation dates (found no file in my collection where these two were not the same).

I've also added a testcase for mkv metadata parsing and updated the bit rates for the other testcases.

tldr: before this PR any mkv files in my library would use the filesystem modification time, now the embedded creation time is used.

[bpatrik]

Thank you for the clean PR.

This might also help with #682 (need to check)