Add "visitor" plugins support (#1454)
Visitor is a simple pattern which helps to avoid many type checks and provide both "perItem" and "perItemReverse" functionality without fragmentation. The most important case is an ability to define state which in many plugins specified either on module level or by polluting `params`. In this diff I added visit and detachFromParent utilities and refactored new mergeStyles plugin with it. Also fixed bug when cdata content is merged into "text" node which is not always valid.
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