Add basic ExtensionPoint support.

Shared/Extensions/SmallIconProvider.swift

@@ -0,0 +1,42 @@
+//
+//  SmallIconProvider.swift
+//  NetNewsWire
+//
+//  Created by Brent Simmons on 12/16/17.
+//  Copyright © 2017 Ranchero Software. All rights reserved.
+//
+
+import Foundation
+import Articles
+import Account
+import RSCore
+
+protocol SmallIconProvider {
+
+	var smallIcon: IconImage? { get }
+}
+
+extension Account: SmallIconProvider {
+	var smallIcon: IconImage? {
+		if let image = AppAssets.image(for: type) {
+			return IconImage(image)
+		}
+		return nil
+	}
+}
+
+extension WebFeed: SmallIconProvider {
+
+	var smallIcon: IconImage? {
+		if let iconImage = appDelegate.faviconDownloader.favicon(for: self) {
+			return iconImage
+		}
+		return FaviconGenerator.favicon(self)
+	}
+}
+
+extension Folder: SmallIconProvider {
+	var smallIcon: IconImage? {
+		AppAssets.masterFolderImage
+	}
+}