Adds a filename split seperator setting that lets you choose between old style `_` and `.` because dot is the only char that isn´t allowed in playername but in texturenames.
Default value is `_` to keep it compatible with older versions, while `.` offers a solution to fix #54.
Check for core feature `httpfetch_binary_data` to run the updater properly.
This requires Minetest cb00632 or newer.
README.md: Add documentation for the updater
