Show encoding results, and fix a verify-encoding issue.

It turns out that Matroska files contain some bytes that are generated
at random. This PR changes the VerifyEncoding function to skip those
bytes by using the "vpxdec --md5" tool to verify that there are no changes.

In addition, a flag for showing full encoding results from list_config_results
was added.
Consider putting the most common code path first and the exception later, like

if extension != 'webm':
  return filecmp...
else:
  # Matroska files...