Created a method ``remove_all_except()`` in scene.py and interactive_scene.py, and made default colors easily configurable. (#2346)

* created a method remove_all_except() in scene.py and interactive_scene.py

* Made it such that default mobject colors can be set through the yaml config file.

* * Default color initialisation wasn't working.
Changed conditional expression to `or` instead.

* Added default values to yaml file.

* added set_background_color() function to Scene class

* Changed default font back to Consolas