index.md special handling

[emirotin]

Currently we do special handling for the index.md because of different way it's converted:

index.md -> index.html somefile.md -> somefile/index.html

We do pass custom filename <-> ref conversion functions to the dynamic pages expansion plugin, but we probably don't have to. We should also investigate if we can drop it altogether, or only keep it in the nav parsing part.

Separate question: would we ever need dynamic index page?