Enable omitting system notes in get_order_notes()

Hi,

Thanks for your great plugin!

I’ve noticed that inside get_order_notes() you search for: customer (sent to customer), private (not sent to the customer) and all (both customer and private) notes.
It would be great if you could add a check to this function or at least output a DIV class when a note is made by the system (like informing about lowering the stock of products). This way we could fetch only those private notes, which we made ourselves. Currently the private notes contain both self-made and system notes. WordPress has a different CSS class in the admin panel for system notes, so this should be easy to implement just like they did.

All the best!
Chris Trynkiewicz from http://sukcesstrony.pl