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)

Why do we need to access a private property here? Generally a leading underscore denotes a private field that is not part of the public API and is thus not guaranteed to be stable to library upgrades.

If you're looking to get the URL/query params from `this.props` — which is definitely the right way to go (vs. from `window.location`) — you can dig into the `Page` class and you should be able to pass in `this.props.query` as a prop to `<Sidebar />`. I haven't tested this though.