[BottomNavigation] Add component prop

[lychyi]

• I have followed (at least) the PR section of the contributing guide.

closes #13953

This allows BottomNavigation to use anything as its root node. In the use case described in #13953, we can now use the Paper component to add elevation.

// Render a BottomNavigation with some elevation by replacing the root div with Paper
class ElevatedBottomNavigation extends React.Component {
  paperComponent = props => <Paper elevation={this.props.elevation} {...props} />
  render() {
    return <BottomNavigation component={this.paperComponent} elevation={5} />
  }
}

[oliviertassinari]

@lychyi It's a great first pull request on Material-UI 👌🏻. Thank you for working on it!