Add server config / server properties

[jellz]

Add a user-friendly config to configure server settings like:

• difficulty
• default gamemode
• host IP
• host port
• max build height
• online mode?
• max players
• pvp?
• command blocks?
• motd
• spawn protection
• mob spawning?

~~I'm not sure what the format will look like yet.~~ It would use the .properties format.