Add an option to disable auto adding class

Sometimes, active, first, last class is not really necessary for a list, such as
![screenshot from 2013-09-15 16 43 59](https://f.cloud.github.com/assets/2468952/1144935/fd0e1df6-1de2-11e3-830c-706d9cc7bd5c.png)
So I think it's appropriate to have an option to disable this behavior.