Add anchorButton and anchorButtonClass options

Inspired by "Open in New Window" PR https://github.com/daviferreira/medium-editor/pull/280.

This one adds two options to the editor:
- ```anchorButton```: (boolean) enables/disables option to add class ```anchorButtonClass``` to anchor tags
- ```anchorButtonClass```: (string) class to add to anchor tags, when ```anchorButton``` is set to true

When you set ```anchorButton``` to ```true```, you will see a new option in an anchor form:

![screen shot 2014-09-12 at 17 46 43](https://cloud.githubusercontent.com/assets/312938/4252490/927f125c-3a96-11e4-8025-e90d4a404c6c.png)

If you check this new option "Button", class ```anchorButtonClass``` is added to the created link and it creates a styled button:

![screen shot 2014-09-12 at 17 46 49](https://cloud.githubusercontent.com/assets/312938/4252645/da6153aa-3a96-11e4-9bc1-3678ae726372.png)