Add JFXX Support

[Friendseeker]

Some images embed thumbnail using JFXX format, as documented by https://www.ecma-international.org/wp-content/uploads/ECMA_TR-98_1st_edition_june_2009.pdf section 10.1

It seems that JFXX is only used by a small subset of jpegs though. I made a WIP implementation here: https://github.com/Friendseeker/jay-peg/tree/JFXX-support and if someone need to parse JFXX, feel free to fork the branch and complete the implementation