Definition lists

[Aaron Swartz]

Thanks to Michel for Markdown Extra. I propose the definition list
syntax be fixed such that:

Bozo the Clown
   :a clown often known for his humorous antics
    and magical ability to be in multiple places at once
   :an extremely silly way of doing things

creates:

<dl>
<dt>Bozo the Clown</dt>
    <dd>a clown often known for his humorous antics
    and magical ability to be in multiple places at once</dd>
    <dd>an extremely silly way of doing things</dd>
</dl>

The space-before-colon looks much more elegant to me and having
multiple definitions seems pretty essential. (The space-after-colon
system can continue to work; I just personally think it's ugly.)