Created access point for guiFactories for external registering.

It was either this, or an event fired after finishMinecraftLoading() to use it to reflect in and add my own values.

*TLDR at the bottom.*

**The reason:**
I've been designing a config annotation system, which some might deem unnecessary, that's fine.  
It can do a lot, and is easily expandable.
I can go into more detail for the annotation system.  
Regardless, if you want to understand more, you can go [here](https://github.com/Jezzadabomb338/OmnisCore/tree/master/common/me/jezza/oc/api/configuration).  
I know that some people dislike the idea of the system for their own reasons, I couldn't care less.  
The point of this edit is that my config system is smart. Smart enough that it should be able to create a default GUI with all of it's knowledge about the mod's configuration needs.

If you need me to go into more detail... 

**TLDR: I need access to register a default GuiFactory for all the mods that use my system.**