Improve validation of inter links

**Don't merge this before we fix #54.**

For `index.md` we want that if the title of `??-*.md` is `Foo Bar` than we use **only** `[Foo Bar](??-*.md)`. For `??-*.md` we don't want to force this rule.
> In my understanding of the spec, we have a strong preference towards checking the text of links. It should not be disabled by default in most validators.

I disable it because we validate **all** Markdown files. With it enable I will need to use

~~~
At [Topic Title One](01-one.html) ...
~~~

instead of

~~~
[Previous](01-one.html) ...
~~~

IMHO we only need to enforce the first form at `index.html`.

> We *could* change the header to:

+1 for that.