Use explode-path not string munge in post-path->link.

I'm committing this from OS X, will pull and test on Windows.
Use build-path in tests, to work on Windows, too.
abs->rel/www always converts to Unix style path.

And this way, post-path->link can simply call it first, then check the
resulting string for an "/index.html" suffix.
On Win line buffer so verbosity messages appear.
Bump to v0.21
Merge pull request #99 from greghendershott/issue-98

Fix issue #98