position='bottom' should use bottom in style, not top

[fungilation]

https://github.com/crazycodeboy/react-native-easy-toast/blob/master/index.js#L105

This line means I have no way to control layout of the toast to have a constant positioning of X pixels away from the bottom of screen, as measured from the bottom of the toast. When position='bottom', the style logic should switch to using bottom: {positionValue} instead of top.