allow easier override of default plugins

[mockersf]

Somewhat related to #65

My use case is to allow the use of a different AssetPlugin. With this PR, I can do:

let mut default_plugins = bevy::DefaultPluginBuilder::default();
default_plugins.with_custom::<bevy::asset::AssetPlugin>(|builder| {
    // setup my custom plugin replacing `AssetPlugin`
});
App::build()
    .add_default_plugins_with_builder(default_plugins)
    .run();

[cart]

I like this, but I think I'd prefer it if it was a bit more generic. I put together an alternative that I think covers your use case, but also allows us to re-use it for other "plugin groups".

#744

[cart]

I'm closing this for now, but I'm happy to re-open if you want to discuss this further.