Add macOS support (#23)

* Add macOS support

* Update README

* Update to v0.2.0

* Document breaking changes

* Delete widget tests

example/macos/Runner/AppDelegate.swift

@@ -0,0 +1,13 @@
+import Cocoa
+import FlutterMacOS
+
+@main
+class AppDelegate: FlutterAppDelegate {
+  override func applicationShouldTerminateAfterLastWindowClosed(_ sender: NSApplication) -> Bool {
+    return true
+  }
+
+  override func applicationSupportsSecureRestorableState(_ app: NSApplication) -> Bool {
+    return true
+  }
+}