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?
This run in all files.

1. I don't hope that people use definition list outside `reference.md`.
2. I don't see problem that the items at definition list outside `reference.md` have the `id` attribute.
3. I want to avoid add complexity to `Makefile` since Pandoc's JSON format doesn't have information about the file name:

~~~
$ cat sample.md 
---
foo: bar
---
$ pandoc -f markdown -t json sample.md 
[{"docTitle":[],"docAuthors":[],"docDate":[]},[{"Table":[[],["AlignDefault"],[0],[[]],[[[{"Plain":[{"Str":"foo"},{"Str":":"},"Space",{"Str":"bar"}]}]]]]}]]
~~~