Allow multiple enque same time, remove sealed

[epsi1on]

Hi,
Thanks for good library. I have two suggestions:
1- Allow developer to enqueue multiple items at same time (by ObservableConcurrentCollection<T>.EnqueueItems(IEnumerable<T> items)). Note that user can already enqueue items one-by-one but for some high performance stuff, if multiple items are added then the collection change can be notified for an array instead of one-by-one. thus the CollectionChanged will not be fired for each enqueue.
2- remove sealed keyword from ObservableConcurrentCollection<T> so that user can inherit from it.

Thanks again

[YounesCheikh]

Thanks for the suggestion, it seems to be a good improvement, I'll take a look to see how this could be implemented.