Add support for AssetMode::Processed

[jf908]

I wanted to use this plugin with AssetMode::Processed with PluginMode::ReplaceDefault so I made these minimal changes to get it to work.

The default asset folder is selected based on whether an imported_assets folder exists or not. Ideally it would be selected whether AssetMode::Processed is set but I don't think can be known at compile time.

[mockersf]

thanks!