Proposal for issue3

Proposal for #3
I moved NbDocument.runAtomic into EmmetEditor.
Unfortunately, Undo/Redo behavior is changed. 
e.g. 
(old) ul+ (expand) ->`<ul><li></li></ul>` -> undo ul+ 
(new) ul+ (expand) ->`<ul><li></li></ul>` -> undo (empty) -> undo ul+ 

I'm sorry if I'm wrong.

Thanks.