created plugin interfaces and plugin manager
- removed variadic template functions. it would've gotten pretty ugly
  and harder to grok
- created a singleton plugin manager to deal with plugins. need to
  create a unified singleton interface
- moved source and header files in Src and Inc directories
- put TemplatePlugin in it's own namespace
fixed issue with WITH_SDL not being used appropriately
- moved Scream build outputs to Binaries/Publish. makes it more obvious
  what is needed to distribute the game