Refactored subchunk handling to allow for more than just "fmt " and "data" - also added "unknown" event emission for unknown chunks

Signed-off-by: Chris Roberts <chris@naxxfish.eu>
Added support for EBU BWF files ("bext" chunk)

Signed-off-by: Chris Roberts <chris@naxxfish.eu>
Added cart chunk support and tests

Signed-off-by: Chris Roberts <chris@naxxfish.eu>