Header: add subheader shorthand

[levithomason]

This is the current markup for Headers with subheader children:

<Header as='h2'>
  Sale!
  <Header.Subheader>
    $10.99
  </Header.Subheader>
</Header>

In keeping with other component shorthand, we ought to allow the Subheader to be configured via props:

<Header as='h2' content='Sale!' subheader='$10.99' />

As with all shorthand props, we should disallow() children when using the subheader shorthand. There are already docs, examples, and tests for Header icon and image shorthand that can be referenced.