Added highlighting for the currently selected tag

Fixes #11 

Opted for a light blue color, which I don't think is too intrusive. I've attached a screenshot of how it looks when the mouse is hovering over it, as well as it's normal color.

![dark-highlight](https://cloud.githubusercontent.com/assets/2644780/3436215/16cbaa9e-009e-11e4-8903-7d9739ed3f64.png)
![light-highlight](https://cloud.githubusercontent.com/assets/2644780/3436216/16cc453a-009e-11e4-83cb-3f981645e62b.png)

You can use a classSet for this:
```
var cx = React.addons.classSet;
var classString = cx({
  "tag-link": true,
  "highlihted-tag": selectedTagId === tag.id,
});
```
http://facebook.github.io/react/docs/class-name-manipulation.html
Not really necessary, but it's cleaner in my opinion