Colorize mutable vars

I think it's useful for some users. It's black by default, so nobody will be disturbed. 

It may look like this:

![image](https://f.cloud.github.com/assets/873919/2491198/2c8879c4-b1df-11e3-9126-79e1490d8f6f.png)

or like this:

![image](https://f.cloud.github.com/assets/873919/2491184/9ae377ee-b1de-11e3-805c-a26f180a39a4.png)