Make 'details' tags work

[Chris Warburton]

The <details> element lets us hide extraneous detail, which I could make
a lot of use of. However, the current Pandoc version doesn't like it. If
I write something like:

    Foo

    <details>
      <summary>Bar</summary>

    Baz

    </details>

    Quux

We end up with HTML like this:

    <p>Foo</p>

    <p><details><summary>Bar</summary></p>

    <p>Baz</p>

    <p></details></p>

    <p>Quux</p>

Notice that the opening and closing details tags are wrapped in p tags,
which is not valid HTML. Thankfully our test suite catches this and
aborts the rendering. Note that it doesn't seem to do this with div
tags, which I've used before to do collapsible content (using jQuery).

There seem to be two ways forwards: try upgrading Pandoc to see if this
is fixed, or add a post-processing step to the 'cleanup' script to find
these broken tags and strip off the 'p' wrappers.