script/generate is documented to find generators in plugins, but it ignores configuration.plugin_paths, and so will only find generators in plugins that are in vendor/plugins. This patch fixes that.
Notes and possible points of discussion:
1. The generate script loads environment.rb, and later looks for generators; by the time the generator search is performed, the initializer and configuration objects are long gone. Either the generation framework has to hook in and be called to perform the search during the initialization process, or the configuration object has to be saved somehow. I opted for a module attribute Rails.configuration.
2. During typical Rails startup, Rails::Initializer.run is called at least twice, with different 'command' parameters. Rails.configuration is set at the end of the process method, so that it will only be set with the Configuration instance that was used to actually configure the environment.
3. This patch slightly changes the way plugin-based generators are reported in the script/generate usage message.
