Feature: prop for passing function when close button is clicked

**Do you want to request a *feature* or report a *bug*?**
Feature
**What is the current behavior?**
The current behaviour is that I have to create a new button so I can pass an onClick listener, but the one by default its fine.  

Would be nice to add a new prop like: 
onButtonClose | function | Called when the notification close button is clicked.
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