Additions

• Implement #16 - Disable PVPToggle in certin worlds

Fixes

• Fix broken PvP vulnerable placeholder by @JerichoTorrent in https://github.com/LukeOnuke/pvp-toggle/pull/14
• Fix #15, /pvp now displays the apropriate state.
• Fix #13, placeholder is now functioning.
• Fix #17 - Do not display space before messages if prefix is empty

Refractorings and code

• Changed the way configuration is read, now with a single service ConfigurationService.
• Added documentation to all of the service classes and their methods inside com.lukeonuke.pvptoggle.service.
• Clarified code