remove bottom value for Customtitle style

Hi 

Thanks for the great library. 

Please remove the bottom value of Custom style, so we can control the whole navbar, i would like to add progress bar to navbar.

```
customTitle: {
    position: 'absolute',
    left: 0,
    right: 0,
    bottom: 7, // --- change this to 0
    alignItems: 'center',
  },
```