Auto index page generator

I implemented the feature which generate index pages automatically in the directories which does not have `index.html`.

For example:

```dart
main() {
  new Generator(rootDir: PROJECT_DIR)
    ..withExamples = true
    ..withIndexGeneration = true
    ..generate(doCustomTask: (workDir) {
      moveExampleAtRoot(workDir);
    });
}
```

This script will generate gh-pages and every directories have `index.html`.

This could be easier with `.where((File f) => f.existsSync())` but it's ok with this async code. You can also use `asyncExpand(_removeIfExests)`.