Expose list nesting level in README and demo toolbar.

Hi,

Firstly, thanks for sharing a great library. 

I've integrated Squire with an iOS app I'm building, which needs to support list indentation. I went to see if I could add it to Squire and noticed that the functionality's there, but not documented. This PR addresses that in case this was an oversight, but if the feature's not yet ready, is there anything I can help with to get it to that stage?

After playing around with it the only I spotted is that it's currently possible to nest the first item in a list, which I'm not sure is desirable, e.g.
![screen shot 2014-12-11 at 12 41 04](https://cloud.githubusercontent.com/assets/265907/5394273/490c4fb6-8133-11e4-86e5-e4d3511f1ada.png)

I'll raise an issue if this is a bug (and perhaps try to fix it)?