Support for top-down file ordering, automatic indentation

I have added an option for top-down ordering of included files. This feature is useful for working with <a href="https://angularjs.org">AngularJS</a>, where one can create a module somewhere at the project directory structure, and then include into such module components separated into small *.js files, nested in subdirectories, without the need to create a new module in each component's file.

Simple illustration:

/project/feature/feature.js
```js
(function(){
  // we create angular module 'project.feature'
  var module = angular.module('project.feature', [ /* dependencies */ ]); 
})();
```

/project/feature/controllers/SubfeatureCtrl.js
```js
(function(){
  // we just extend existing module, since with proper <script src="..."> ordering in our html
  // we have a guarantee that this module was already created at this point
  angular.module('project.feature').controller('Subfeature', [$scope, function ($scope) {
    /* ... */
  }]);
})();
```

/project/feautre.html
```html
<!doctype html>
<html>
  <head>
    <title>Feature of the project</title>
  </head>
  <body>
    <!-- include: "type": "js", "files": "/project/**/*.js", "ordering": "top-down" -->
    
    <!-- files below will be included automatically by grunt-include-source task -->
    <script src="/project/feature.js"></script>
    <script src="/project/feature/controllers/SubfeatureCtrl.js"></script>
    <!-- ... -->
  </body>
</html>
```

In addition I have fixed indentation for included files (<a href="https://github.com/jwvdiermen/grunt-include-source/issues/14">issue #14</a>).