Add ability to run configurable "preupdate" and "postupdate" shell scripts during image file writing.

Preupdate runs before replacing the .png, postupdate runs after. Both wait for completion before continuing, so scripts should be fast, or fork into the background themselves if appropriate. The image file name is provided as an argument.

Expected use-cases:
-Run optipng or similar in preupdate
-Upload to webserver or flush caches in postupdate

I'm sure there are others too. :)
Make sure updated JSON files are fresh (no extra)
Remove Java 7 specific code in Maeyanie's PR
Switch to more structured configuration settings for image update exits
Prevent overlapping file updates on JsonFile update
Start Pam's Mods Support