feat: allow to change background for FAB Group (#758)

### Motivation

It's currently possible to change the background color for FAB and also items within FAB.Group components. Changing the background color in FAB Group causes the overlay background to change.

This PR makes it possible to change the background for the FAB Group main button.

Current behavior:
<img width="377" alt="screen shot 2019-01-25 at 17 41 56" src="https://user-images.githubusercontent.com/7827311/51761486-797b0280-20cd-11e9-93de-1eaf1ba64ff8.png">

### Test plan

https://snack.expo.io/ryjNEaO7E

src/components/FAB/FABGroup.js

@@ -71,6 +71,10 @@ type Props = {|
    * For example, you can set an additional margin if you have a tab bar at the bottom.
    */
   style?: any,
+  /**
+   * Style for the FAB. It allows to pass the FAB button styles, such as backgroundColor.
+   */
+  fabStyle?: any,
   /**
    * @optional
    */
@@ -198,6 +202,7 @@ class FABGroup extends React.Component<Props, State> {
       accessibilityLabel,
       theme,
       style,
+      fabStyle,
       visible,
     } = this.props;
     const { colors } = theme;
@@ -312,7 +317,7 @@ class FABGroup extends React.Component<Props, State> {
           accessibilityTraits="button"
           accessibilityComponentType="button"
           accessibilityRole="button"
-          style={styles.fab}
+          style={[styles.fab, fabStyle]}
           visible={visible}
         />
       </View>

typings/components/FAB.d.ts

@@ -21,6 +21,7 @@ export interface FABGroupProps {
   onStateChange: (state: { open: boolean }) => any;
   visible?: boolean;
   style?: any;
+  fabStyle?: any;
   theme?: ThemeShape;
 }