Allow for skipping unchanged files when running bin/oop2json

[clemens-tolboom]

For performance reasons it would be green to skip unchanged files after ie a git pull on the source tree at hand.

For this to work we could timestamp output file same as input file.

• [ ] Reuse index file
• [ ] Extend index file with time stamps

This is trickier for #35 json2dot as #25 + #27.