Added support for JBIG2 extension segment

This addresses an issue #3616, https://github.com/mozilla/pdf.js/issues/3616. The extension segment (62) was not being handled in the switch/case so the default case threw an exception. Per the JBIG2 spec (7.4.14-7.4.15) this information is simply a metadata comment and per the PDF32000 spec (7.4.7) can be safely ignored.

JBIG2 spec - http://www.jpeg.org/public/fcd14492.pdf
PDF32000 spec - http://www.adobe.com/content/dam/Adobe/en/devnet/acrobat/pdfs/PDF32000_2008.pdf