Markdown reader: Fixed small bug in HTML parsing with markdown_attribute.

Test case:

    <aside markdown="1">
    *hi*
    </aside>

Previously gave:

    <article markdown="1">
    <p><em>hi</em> </article></p>
Fix behavior of `markdown_attribute` extension.

It now works as in PHP markdown extra.  Setting `markdown="1"` on
an outer tag affects all contained tags until it is reversed with
`markdown="0"`.  Closes #1378.

Added `stateMarkdownAttribute` to `ParserState`.