Make headers optional.
Fix typo.
Refactor to retrieve message parts.
Handle exit codes/exceptions/stderr logging, improve outputDir handling.
PDF files with original filename - closes: #8, #9

Also test PDF attachments.
Add test for when a PDF already exists.
Extract all PDFs, not just one - closes #7
Add tests with no headers.
Refactor tests with a common superclass so output file can be checked.