adding badgeStyle props

[cyril36]

Hello,

I have added a badgeStyle props as commented in the issue #67

I wanted to include all the badgeProps in this props as in the example :
badgeProps={mybadgeProps}

but i dont know what kind of interface type i should use :

export interface TabScreenProps {
   label: string;
   icon?: IconSource;
   badge?: string | number | boolean;
   badgeProps?: ???;
   children: any;
   onPress?: (event: GestureResponderEvent) => void;
   onPressIn?: (event: GestureResponderEvent) => void;

I dont know much Typescript so if you want me to add more props and not just the style i can do a new PR later but i would appreciate if you can indicate me the type of the props i should put.

Meanwhile i hope this PR will be useful and thank you for this nice library ;)

[cyril36]

After taking a break the obvious come to my mind and knew how to create this badgeProps and felt stupid about asking :D

I have updated the badgeStyle to badgeProps without including children as you already do it with the badge prop.
I also kept your default value if the size is not mentioned in the badgeProps.
I have also updated the readme with how to use the badgeProps prop
Let me know if any update is needed