Issue description

• version #4.0.1
• components: Nav

Steps to reproduce issue

Just create a <Nav stacked>...</Nav> component => items are not stacked.

In code we see it results in nav-stacked class being added: https://github.com/reactstrap/reactstrap/blob/master/src/Nav.js#L41

This produces no desired affect as current BS4@alpah6 documentation suggests adding a general 'flex-column' utility class:
http://v4-alpha.getbootstrap.com/components/navs/#vertical