HTML reader:  improved handling of tags that can be block or inline.

Previously a section like this would be enclosed in a paragraph,
with RawInline for the video tags (since video is a tag that can
be either block or inline):

    <video controls="controls">
       <source src="../videos/test.mp4" type="video/mp4" />
       <source src="../videos/test.webm" type="video/webm" />
       <p>
          The videos can not be played back on your system.<br/>
          Try viewing on Youtube (requires Internet connection):
          <a href="http://youtu.be/etE5urBps_w">Relative Velocity on
    Youtube</a>.
       </p>
    </video>

This change will cause the video and source tags to be parsed
as RawBlock instead, giving better output.

The general change is this:  when we're parsing a "plain" sequence
of inlines, we don't parse anything that COULD be a block-level tag.