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.

>   * Most links across any markdown file should NOT have their text
>     checked by default
>

If it looks like a link to a local file, it should be checked - if it 
starts with a protocol prefix (e.g., `http://`) then it shouldn't be.

>   * ALL links in index.md should have their text checked, no matter
>     where in the document the link resides.
>   * Links in reference.md should be checked, but only if they are in
>     an h2 level header
>
+1 to rules 2 and 3.

Thanks,
Greg