BaseRenderer should be abstract class

[Spenhouet]

Your design for v2 allows for the definition of new renderers.

To implement new classes which should follow some structure and implement some methods it is crucial to have the base class (BaseRenderer) be implemented as abstract class.

Please consider implementing the BaseRenderer as abstract class: https://docs.python.org/3/library/abc.html
I'm also missing something like that for the plugins.

This will make the implementation of new compatible renderers much easier.