Finish the design of Railcraft API for Mc 1.12

[liach]

Once an official rc is released for mc 1.12, the API will be frozen. As a result, we should design it as early as possible with all the precautions possible. @Generalcamo Asking you for participation.

[Generalcamo]

Added support for Secondary Blast Furnace outputs. A breaking change, so was good to get that in now.