[BottomNavigation] Use value over index

[oliviertassinari]

Closes #2957 with the following breaking change:

-        <BottomNavigation index={index} onChange={this.handleChange}>
+        <BottomNavigation value={value} onChange={this.handleChange}>

I believe it's the only API that goes against the suggested one in the issue.

• PR has tests / docs demo, and is linted.
• Commit and PR titles begin with [ComponentName], and are in imperative form: "[Component] Fix leaky abstraction".
• Description explains the issue / use-case resolved, and auto-closes the related issue(s) (http://tr.im/vFqem).