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.

+1.
>
> 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) ...|

We want to be able to validate both: links that are supposed to have 
particular text (e.g., table of contents, h2-level headers in 
`reference.md` back to sections) should have text validated; links in 
other locations shouldn't.