Modify Tab items on runtime

### Version
Tell us which versions you are using:

- react-native-router-flux v4.0.2
- react v16.8.3
- react-native v0.59.9

Is it possible to modify the RNRF Tabs component during runtime? I have to hide some tabs according to user privilege and I can't seem to find out how to accomplish this since my logic kicks in way after the initialisation of the Scenes/Tabs.
I'm having difficulty finding out if this is supported or even stumble upon workarounds.

### Expected behaviour
Support modifying the TabBar during operation and not only on initialization

### Actual behaviour
Unable to find a way to modify the RNRF TabBar items (even just the style) during operation and in the middle of app runtime

### Steps to reproduce
1. Have a functioning base implementation of the RNRF Tabs. Specially as a main bottom navigation option module
2. A scenario where a certain tab must be modified (hidden in my case).