Add props to headerLeft and headerRight

[AzzouQ]

This PR will allow users to custom headerRight and headerLeft components using user's theme color.
For example, if I want a custom settings icon in headerRight to navigate to my settings page, I will be able to retrieve color from my custom theme. It's already available in ReactNavigation.

colors.primary is used by the back button so it makes sense to use it for headerRight and headerLeft.

The same logic may be applied to headerCenter (Imagine a custom TextComponent), we may apply colors.text in this case.

Let me know if you find this useful (or not), or if I can make it better to be integrated.

[WoLewicki]

I think it would be best if you replicated the behavior of color={headerTintColor !== undefined ? headerTintColor : colors.primary}. Also could you add the same thing for the headerCenter?

[AzzouQ]

@WoLewicki Done, I add it to headerCenter with colors.text too. I personally prefer to use ?? operators (as defined here https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Nullish_coalescing_operator) but if you want me to change back to the classic a !== undefined ? a : b, there's no problem.

[WoLewicki]

Please change it to the classic form so we can keep it consistent. Maybe we will change all the occurrences to nullish coalescing soon. Also could you please use the colors.primary in the headerCenter too? It seems more fitting for every custom view to get the same default color.

[WoLewicki]

Or we can leave it as it is, nevermind. Will change headerCenter only and merge it since we are releasing the library.

[AzzouQ]

Wouldn’t it be more logical for headerCenter to mimic headerTitle behavior ? headerCenter will most likely be a custom TextComponent, so colors.text would make more sense ? Anyway, glad it’s getting merged 👍🏼