basic plugin folder support

I've created basic plugin folder support.
This loads the plugins into the app:
```C#
Plugins.Load(app);
```
This is all that is needed to make a plugin, just drop the dll into the %AppDataPath%/plugins folder.
```C#
[Export(typeof(IPlugin))]
[ExportMetadata("Name", "Download History")]
[ExportMetadata("Description", "The quick fox jumped over the lazy dog.")]
//[ExportMetadata("Version", "1.0.0")]
[ExportMetadata("Author", "John Smith")]
[ExportMetadata("Website", "https://example.com/")] 
class MyPlugin : IPlugin
{
		void StartRun(){}
		void StopRun(){}
}
```
I couldn't decide on a nice way to give the plugins access to the various services like database access, events etc. Was hoping you could help me with that.