Plugin overriding

[cart]

When initializing an application, most users will probably make a call to add_default_plugins(). This call results in the immediate registration and initialization of all default plugins, and there is no way to unregister or modify the plugin after the fact. It should be possible for users to do any of the following actions:

• opt out of one or more default plugins, without taking on the responsibility of registering all of the default plugins. (what happens if we add a new default plugin and they forget to add it)
• change the settings of a default plugin, without taking on the responsibility of registering all of the default plugins. (ex: configuring the default render graph settings in RenderPlugin)

Ideally whatever solution we choose applies to any set of plugins, not just the default ones. It might even be worth formalizing "plugin groups".