Add basic ExtensionPoint support.

Shared/ExtensionPoints/ExtensionPoint.swift

@@ -0,0 +1,20 @@
+//
+//  ExtensionPoint.swift
+//  NetNewsWire
+//
+//  Created by Maurice Parker on 4/7/20.
+//  Copyright © 2020 Ranchero Software. All rights reserved.
+//
+
+import Foundation
+import RSCore
+
+protocol ExtensionPoint {
+
+	/// The title of the command.
+	var title: String { get }
+
+	/// The template image for the command.
+	var templateImage: RSImage { get }
+
+}