Don't store configuration in the App_Plugins folder

[arknu]

If you're using WebDeploy to publish your site from Visual Studio, the entire App_Plugins folder will be copied to the server, no questions asked. This means that the configuration will be overwritten.
The PageNotFoundManager.config file should go either in App_Data or the Config folder, not in App_Plugins

[TimGeyssens]

Ah good catch, will update

[PhilDye]

Beware of creating a breaking-change for existing installs if the path is moved; it should perhaps fall-through from one path to another?

[ThumNet]

I don't know is this feature request is still current, but I made pull request (#14) for this.