Created chained button group component

Summary:
This is a new component that allows for chaining of buttons in a button group by placing an icon in between them.

This is how we plan to navigate between views within an Android app.

Reviewed By: danielbuechele

Differential Revision: D16071234

fbshipit-source-id: 404a3a341081b6111667bd5acf27876bec627435

src/ui/index.js

@@ -12,6 +12,7 @@ export {
   default as ButtonNavigationGroup,
 } from './components/ButtonNavigationGroup.js';
 export {default as ButtonGroup} from './components/ButtonGroup.js';
+export {default as ButtonGroupChain} from './components/ButtonGroupChain.js';
 
 //
 export {colors, darkColors, brandColors} from './components/colors.js';