Implemented caching!

Changelog

• Added a config:
  • verboseUpdateMessages: true
    • Set to true you want more messages telling you what the plugin is up to.
  • cacheHours: 72
    • How long to keep the playerheads cached for. The lower this number, the faster skin changes will appear, but the more network usage there will be.

This is still fairly untested, so please tell me if there are any bugs or general things that don't work nicely!