Add renderBackButtonIcon for backButtonIcon customization

[Kudo]

08dc150 is good if we would like to customise all the logic of backButton. However, there are some good part of internal logics, e.g. https://github.com/exponentjs/ex-navigator/blob/master/ExRouteRenderer.js#L94-L113. This change proposes a way to only customise the BackIcon component from ExNavigator. Please feel free to comment and we should discuss here. Now I think there are too many options (propTypes) in ExNavigator interface. While enabling renderBackButtonIcon, barButtonIconStyle is useless and not sure if that would be implicit or not.

[mention-bot]

By analyzing the blame information on this pull request, we identified @ide, @jesseruder and @brentvatne to be potential reviewers

[ahanriat]

@Kudo thanks for this PR, hope it will be merged soon as we currently have a fork for the same reason :smiley:

[magrinj]

:+1: This is really a need!

[Emilios1995]

This is useful, hope it gets merged.