dejsonlz4: Decompress Mozilla Firefox bookmarks backup files

Current Firefox bookmarks backup files are stored as non-standard file
format based on lz4 compression. These files have a .jsonlz4
extension. Use dejsonlz4 to decompress them.

lz4.c and lz4.h at this repository are verbatim copies from the
Mozilla repository as of 2016-05-12 (as currently used by Firefox)
[1].