Add examples (#43)

- How to run the animation between 2 specifics frames
- How to export the animation to a file

example/lib/examples/specific_frame.dart

@@ -0,0 +1,45 @@
+import 'package:flutter/material.dart';
+import 'package:lottie/lottie.dart';
+
+void main() => runApp(MyApp());
+
+class MyApp extends StatefulWidget {
+  @override
+  _MyAppState createState() => _MyAppState();
+}
+
+class _MyAppState extends State<MyApp> with TickerProviderStateMixin {
+  AnimationController _controller;
+
+  @override
+  void initState() {
+    super.initState();
+
+    _controller = AnimationController(vsync: this);
+  }
+
+  @override
+  void dispose() {
+    _controller.dispose();
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      home: Scaffold(
+        body: Center(
+          child: Lottie.asset(
+            'assets/LottieLogo1.json',
+            controller: _controller,
+            onLoaded: (composition) {
+              _controller
+                ..duration = composition.duration ~/ 5
+                ..repeat(min: 0.0, max: 0.2, reverse: true);
+            },
+          ),
+        ),
+      ),
+    );
+  }
+}