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.
Sounds like we'll need further changes before we merge this pull; the first paragraph ("always check") conflicts with the last paragraph ("sometimes check"). If the below heuristic is ok, I'll implement later today...

* Most links across any markdown file should NOT have their text checked by default (per @r-gaia-cs current implementation).
* 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