Make markdown_attribute work like in PHP Markdown Extra and multimarkdown

[bpj]

jgm wrote:
+++ BP Jonsson [Jun 28 14 00:26 ]:

2014-06-27 20:04, John MacFarlane skrev:

Try putting the attribute on the tr and table as well.
I believe the way it works is this: if you hit a block-level tag
without the attribute, then the whole contents of the tag are parsed
as HTML. (That includes any interior tags, whether or not they have
markdown=1.)

That's rather counter-intuitive. I would also have expected that
everything below a block level tag with markdown=1 would
'inherit' the positive value. Why not make it so and then make
markdown=0 significant so that one can turn the feature off
further down? I guess you could keep a stack with values for
'markdownness' at the different nesting levels (as in the
comments below).

[snip original example; improved below]

<table markdown=1>                          <!-- [1] -->
    <tr>                                    <!-- [1, 1] -->
        <td>*This* is markdown</td>         <!-- [1, 1, 1] -->
        <td markdown=0>                     <!-- [1, 1, 0] -->
            <p>This is *not* markdown</p>   <!-- [1, 1, 0, 0] -->
            <p markdown=1>                  <!-- [1, 1, 0, 1] -->
                But this *is* markdown!
            </p>
        </td>
    </tr>
</table>

Well, it turns out this IS the way it works in PHP Markdown Extra
and multimarkdown. And since the point of the extension is to
allow conversion between these formats, I should do it this way,
too.

It would be helpful if someone could put an issue up on the tracker
so I don't forget.