Fix up readmes (#578)

Use ``` for examples - fixes https://github.com/dart-lang/stagehand/issues/577
Drop package name - fixes https://github.com/dart-lang/stagehand/issues/576

CHANGELOG.md

 ## 3.1.3
 
+- All templates: small improvements to generated `README.md`.
+
 ## 3.1.2
 
 - All templates: Set min SDK version to 2.0.0.

lib/src/generators/console_full.g.dart

@@ -21,10 +21,10 @@ b2MvYXBpLwo=''',
   'README.md',
   'text',
   '''
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+U0UpLgo=''',
   'analysis_options.yaml',
   'text',
   '''

lib/src/generators/package_simple.g.dart

@@ -21,15 +21,15 @@ b2MvYXBpLwo=''',
   'README.md',
   'text',
   '''
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   'analysis_options.yaml',
   'text',
   '''

lib/src/generators/server_shelf.g.dart

@@ -21,11 +21,10 @@ b2MvYXBpLwo=''',
   'README.md',
   'text',
   '''
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   'analysis_options.yaml',
   'text',
   '''

lib/src/generators/web_simple.g.dart

@@ -21,10 +21,10 @@ b2MvYXBpLwo=''',
   'README.md',
   'text',
   '''
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+Lgo=''',
   'analysis_options.yaml',
   'text',
   '''

lib/src/generators/web_stagexl.g.dart

@@ -21,10 +21,10 @@ b2MvYXBpLwo=''',
   'README.md',
   'text',
   '''
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   'analysis_options.yaml',
   'text',
   '''

templates/console-full/README.md

-# __projectName__
-
 A sample command-line application.
 
 Created from templates made available by Stagehand under a BSD-style

templates/package-simple/README.md

-# __projectName__
-
 A library for Dart developers.
 
 Created from templates made available by Stagehand under a BSD-style
@@ -9,11 +7,13 @@ Created from templates made available by Stagehand under a BSD-style
 
 A simple usage example:
 
-    import 'package:__projectName__/__projectName__.dart';
+```dart
+import 'package:__projectName__/__projectName__.dart';
 
-    main() {
-      var awesome = new Awesome();
-    }
+main() {
+  var awesome = new Awesome();
+}
+```
 
 ## Features and bugs
 

templates/server-shelf/README.md

-# __projectName__
-
 A web server built using [Shelf](https://pub.dartlang.org/packages/shelf).
 
 Created from templates made available by Stagehand under a BSD-style

templates/web-simple/README.md

-# __projectName__
-
 An absolute bare-bones web app.
 
 Created from templates made available by Stagehand under a BSD-style

templates/web-stagexl/README.md

-# __projectName__
-
 A simple [StageXL](http://www.stagexl.org/) app.
 
 Created from templates made available by Stagehand under a BSD-style