Like the tabs macro, the list macro should get a possibility to add a class parameter

Like the tabs macro, the list macro should get a possibility to add a class parameter.

I did adjust the core list macro and the docs. There is no compatibility problem. 

As shown in the group: https://groups.google.com/forum/#!topic/tiddlywiki/dFvjEH4eqE0 ... if someone wants have a multi column list, we have a 100% hit for an issue.