Implement configurable infection distance

These 2 commits implement the ability for server owners to modify the range for player-to-player infection (option "infection-spread-distance" default: 3). Older config files will be updated with the new configuration option.

Note: Has not been tested yet.

There's also one issue: The way the plugin works will not allow server owners to change the config.yml file while the server is running, thus they must change "infection-spread-distance" when it is offline.