how to move a routeName to another index without remount

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

- react-native-router-flux v4.?.?
- react v16.?.?
- react-native v0.?.?

### Expected behaviour

### Actual behaviour

### Steps to reproduce
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### Reproducible Demo

Please provide a minimized reproducible demonstration of the problem you're reporting.

Issues that come with minimal repro's are resolved much more quickly than issues where a maintainer has to reproduce themselves.