Make filter for id at definitions general

This solves #77 by add id attribute to every item of a definition.

I don't see big problems in

~~~
Foo
:   Some text
~~~

be convert to

~~~
<dl>
<dt><span id="foo">Foo</span></dt>
<dd>Some text
</dd>
</dl>
~~~

in every file of the lesson.

@gvwilson and @abought Comments?
Does this only get run inside `reference.md`, or will it be run in all files?