NBT Exporter

Add more commands:

• /exportnbt exports item NBT only, in compressed binary format
• /exportstack exports full information about the item as NBT (incl. id & count & damage), in compressed binary format
• /exportnbtstr exports item NBT only, in text (stringified) format
• /exportstackstr exports full information about the item as NBT (incl. id & count & damage), in text (stringified) format
• /exportgive exports full information about the item as /give command into .txt

Improve code, add support for simplified filename validation (for Unix OS & FS), lots of smaller fixes and improvements.