Prevent promotion of subtitle to title.

The following code:

    title
    ======
    This line causes the subtitle to become a h1 header.

    Subtitle
    --------
    

Would be rendered with both title and subtitle as h1 headers. This is due to title being a lone section header, hence by default it is promoted to document title and subsequent subheaders are promoted to headers. However, this is inconsistent with how ReST is displayed in many places (including github). This patch fixes this issue.