Use Dynmap's `postupdatecommand` to detect tile changes

[Dantevg]

According to Dynmap's config file, custom-commands.image-updates.postupdatecommand is a

Command run just after any image file is written or updated: run with single parameter with fully qualified file name

Maybe this can be used to detect when no changes have been made at all.

Possible implementation

1. Craft a command that appends the filename of the changed tile to some text file, and let the user add it to Dynmap's config.
2. Whenever DynmapExport starts exporting, read that text file. If none of the tiles correspond with an export, cancel that export.
3. After exporting, clear the text file.