Add ability to pass styles for each individual tab

[greg5green]

Why?

In one of our projects, there is a need to have a different background color for a selected tab (example below).

What changed?

• Added tabStyle and selectedTabStyle props to TabNavigator
  • The correct styles are passed down to Tab based on if the tab is selected
  • I'm only planning on using selectedTabStyle and it seems having a different style per tab is an anti-pattern (as is pointed out in Material Design's Bottom Navigation docs), but I guess configurability is good? ¯\_(ツ)_/¯

[ide]

We merged #73 instead -- similar API but no selectedTabStyle. You should be able to change the style you pass in when you re-render your app upon tab selection.