feat(Header): Add subheader prop

[layershifter]

This PR fixes #461.

[codecov-io]

Current coverage is 98.62% (diff: 100%)

Merging #476 into master will increase coverage by <.01%

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[layershifter]

Seems, I missed test-case there.

[levithomason]

Currently, the Header will render children with shorthand even though there are prop type warnings. While we're here, let's split this return with a conditional return to only render children if provided. The goal in doing this in all components is to enforce proper usage (not rendering shorthand when children are present):

  if (children) {
    return (
      <ElementType {...rest} className={classes}>
        {children}
      </ElementType>
    )
  }

  return (
    <ElementType {...rest} className={classes}>
      {content}
      {subheader && <HeaderSubheader content={subheader} />}
    </ElementType>
  )

[levithomason]

Will merge after the Header render is updated to strictly render children or shorthand, but not both.

[levithomason]

Released in stardust@0.42.2, thanks!