Add first-draft `SubNav` component which renders a link and it's `props.children`, a list
of `NavItem`s.
A `SubNav` is active if one of it's decendants is active
Also add `Affix` component and mixin
Use these new components in docs side nav
Allow active `NavItem` to be set by `activeHref` prop
Rename `isActive` props to `active`
Add space between buttons in docs and upgrade JSXTransformer