[Feature Request] Possibility to give a global customContext object..

... to the default Nelmio onerror logger, without having to implement a custom behavior using the logger function.

Hi there, I'm using this awesome bundle, and the default behavior is just great for me. I quickly noticed that I would need some more extra info sent to the log when an error occurs. By chance, I have on my page an object with all this info I need. But I cannot automatically send it by implementing my own `onerror` function using your `log()` function.

That is the reason I have implemented the ability to define a global param allowing to give this extra info, with the basic implementation.

Hope the idea (an maybe not my current implementation) will be accepted :)

Best