Refactor a bit the StripDown render's tests

Let's write a test for each specific callback and not a single one doing
an assertion on a lot of them. The diff are more readable when dealing
with failures and this will be easier to test new features.
Make the StripDown render object displays links' URL

Displaying only the link's text can be a bit misleading as sentences are
built assuming that the text will be click-able or at least reachable.

Fixes #404.
Merge pull request #405 from vmg/strip-down-links

The `StripDown` render now displays links' URL