Added registerBuildType Method

Added a `registerBuildType` method to allow easier extension of `builder` by plugins. I figure that being able to implement the existing builder methods using this should make it a good starting point. Ultimately it may need some reworking to make it even more extendable, but at least for now it offers a way to add custom asset types, etc. into builder. 

I didn't write tests just because the method is 100% test covered by other tests.