Interpret titles and subtitles in rst

This hopes to make subtitles usable without changing the markup and
table-of-contents generation of documents much at all.

This is an attempt to solve https://github.com/github/markup/issues/278

Previously, titles and subtitles[0] would introduce a lot of nesting
in a generated table of contents.  Enable both `doctitle_xform` and
`sectsubtitle_xform` to remove those.

However, merely enabling those will cause the existing top-level,
non-title HTML header sizes to be promoted to `<h1>`, since titles and
subtitles are not considered "sections", changing how the HTML output
looks a rather lot.

That's not how some sampled rsts (without subtitles) on work today:
the title is biggest at `<h1>`, and all other sections are `<h2>` or
below.  So set the initial_header_level to "2", so that the largest
text sections can have is `<h2>`, tied in size for the subtitle.