Allow onContextMenu even handler (#1956)

Right clicks aren't handled by onClick in a content-editable component. So we need access to a onContextMenu handler to handle these events properly.
My use case is to be able to open a context menu with actions when the user right-clicks a specific inline node.

packages/slate-react/src/constants/event-handlers.js

@@ -8,6 +8,7 @@ const EVENT_HANDLERS = [
   'onBeforeInput',
   'onBlur',
   'onClick',
+  'onContextMenu',
   'onCompositionEnd',
   'onCompositionStart',
   'onCopy',