Replace spurious blockquotes with definition lists.

[Thom1729]

Currently, the spec uses a combination of level-5 headings and blockquotes to emulate a definition list-like layout. But Kramdown supports actual definition lists, so this is unnecessary. Where we can use the proper semantic markup, we should do so.

As a bonus, the headings I replaced were exactly the ones that we didn't want in the table of contents, so that's fixed too.

This PR makes two changes to the perl script. The first is simply to pass through definitions without incident. The second introduces a new syntax for adding anchors. Some of the headings that are now definition terms were used as link targets, so we have to make sure that the definition terms have the same anchors. Kramdown has built-in syntax for this, but the perl script didn't like it, so here I'm using question marks. The following example:

? Scalar

…will be expanded to:

<div id="scalar"/>

Scalar

[ingydotnet]

Personally, I think that ? is clearer (suggestive of YAML dictionary keys) and less likely to be visually confused with a single colon. But it's not a big deal.

? is my least favorite YAML syntax :)

But given that we're rolling our own extensions here, let's give ? a try.