Support attachments as input directly

[richlv]

Is your feature request related to a problem? Please describe.
Would be great to run dmarc_receive against attachments (gz, zip compressed or uncompressed XML).
This would be helpful in debugging/testing scenarios.

Describe the solution you'd like
dmarc_receive --attachment or similar that could detect gz/zip and uncompress, or handle XML directly.

Describe alternatives you've considered
Take the attachment and create a fake email message to contain it.