Implement DHT Merging

The current way multipass works, write_scan_header() seems to be called
multiple times, so only DC tables get merged, however, this will still
merge AC tables if possible.

Implements the second half of #30.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
Merge pull request #97 from dwbuiten/dhtmerge

Implement DHT Merging