Issue title: Dropdown ignores initial selection
 Issue body: Dropdown component does not honor the initial selections specified by "selected" attribute in the option element.

Sample here: http://codepen.io/dyepishin/pen/xggPdE
 Comments: 
 Comment 0: Hey @dmitriyyepishin, thanks for reporting this!

The Dropdown currently does not support setting an initial selection value, but this seems like pretty basic and valuable functionality to add. We'll look into adding either a method to control this when instantiating Dropdowns, or enabling subscribing to the native `select` menu's `selected` attribute. Unfortunately, we can't give a timeline for this, but will look into adding it in a future sprint. I'll leave this issue open until it's been addressed.
 Comment 1: Hi,
I just added this code inside the ctor of Dropdown.ts
```
        if(option.selected){
          newItem.classList.add(IS_SELECTED_CLASS);
        }
```
inside the foreach where the options are created. 

Have a look inside my branch: https://github.com/seriewe/office-ui-fabric-js/tree/dropdown-selected-fix

 Comment 2: Microsoft no longer supports this content and will not be responding to bugs or issues. We recommend that you use the newer version, [Office UI Fabric](https://developer.microsoft.com/fabric), with React as your front-end framework. We are closing this issue; if you still need assistance with Fabric.js, visit [Stack Overflow/office-ui-fabric](https://stackoverflow.com/questions/tagged/office-ui-fabric).
{'source': 'GithubIssues', 'repo_name_with_owner': 'OfficeDev/office-ui-fabric-js', 'issue_id': 201730424, 'repo_stars': 400.0}