Use H1 for header title and cover image caption

[gemedet]

This allows markup inside the header title and cover image caption, e.g.:

<header>
    <figure>
        <figcaption>Some caption <i>in italic</i></figcaption>
    </figure>
    <h1>Some title <b>in bold</b></h1>
</header>

Follows facebookarchive/facebook-instant-articles-sdk-php#93

[everton-rosario]

Hello @gemedet !
This was a nice catch.