glbdiff: diff the json in .glb files.

Usage: glbdiff old_file new_file
       glbdiff --textconv file            [textconv mode]
       glbdiff --git ...                  [git mode]

Integrating with git:

    1. Add the following line to your .gitattributes:

        *.glb diff=glbdiff

    2. Add either of the following options to your git config:

        [diff "glbdiff"]
            textconv path/to/glbdiff.py --textconv

        [diff "glbdiff"]
            command path/to/glbdiff.py --git

Using the 'textconv' option is recommended; it will allow git's own
diff algorithms, colouring etc. to operate as usual; When using the
'command' option, git will just display the output from glbdiff.py
verbatim.