Included jsoneditor.css as a Main Component in Bower

Added the css file to bower so that the css will be identified as a main component used. Making this change allows gulp to identify the css and inject it into the project automatically at build time.

Executing the following shell commands:
```bash
    yo gulp-angular
    bower install jsoneditor --save
    gulp build
```
will result in the index.html automatically being injected as seen below:

```html
  <head>
    <!-- bower:css -->
    <link rel="stylesheet" href="../bower_components/jsoneditor/jsoneditor.css" />
    <!-- endbower -->
    <!-- endbuild -->
  </head>
  <body>
    <!-- build:js scripts/vendor.js -->
    <!-- bower:js -->
    <script src="../bower_components/jsoneditor/jsoneditor.min.js"></script>
    <!-- endbower -->
    <!-- endbuild -->
  </body>
```