Allow custom levels to be defined

[blevine]

I'd like to be able to define additional levels (and associated styles) to be added to the default set of levels (success, error, warning, info)

[miketierney]

I'd like to be able to do this as well; while the default set covers the bases pretty well, having the ability to add a generic style (or simply an additional style) is helpful. For example, I have an implementation where I need to be able to show success, warning, and info as parts of the UX. Additionally, though, I need a "pending" state while I'm waiting for something to process I need to show the user that the action is running. This should be a neutral style.

[MomchilGorchev]

Yes, very common use case - show notification with loading/pending state and then success/failure. Maybe the ability to pass any string for the level prop would be very useful. This will allow custom notification-loading, etc. elements