Support for window transparency

Is it possible to set the window background to transparent and then allow the transparency level to be controlled with CSS? Similar to TextMate?

![screen shot 2014-06-05 at 9 46 46 am](https://cloud.githubusercontent.com/assets/82437/3188124/db02aca0-ecb7-11e3-97ae-aa5f3cf6eaec.png)

/cc @zcbenz 
