Ignore encrypted documents
Original request:
        I am sending multiple pdf, word etc. attachments in one documents to be indexed.

        Some of them (pdf) are encrypted and I am getting a MapperParsingException caused by org.apache.tika.exception.TikaException: Unable to extract PDF content cause by
        org.apache.pdfbox.exceptions.WrappedIOException: Error decrypting document.

        I was wondering if the attachment mapper could expose some switch to ignore the documents it can not extract?

 As we now have option `ignore_errors`, we can support it. See #38 relative to this option.

Closes #18.