Sidbar Nav position

Add a prop so the sidebar nav component can be used either left or right side of the main content.

```jsx
<SidebarNav position='left'>
    <SidebarNavItem selected>
      General settings
    </SidebarNavItem>
    <SidebarNavItem>
      Users
    </SidebarNavItem>
    <SidebarNavItem>
      Teams
    </SidebarNavItem>
</SidebarNav>
```