Use TextStyle for iconStyle

[peterkuiper]

Fixes #53

[himelbrand]

Thanks a lot for your contribution!
Please make it possible to use one or the other... depending on need

[jarod-legault]

@himelbrand I'm not sure why there should be an option of different types for iconStyle. If you look at IconProps from react-native-vector-icons/Icon, it extends TextProps from react-native, which in turn uses TextStyle for the style prop. It seems like this PR is correct and will eliminate Typescript errors for users, specifically when attempting to set the color of the icons. But maybe I am misunderstanding your comment.