Ignore foo.html when foo.scrbl exists in _src/posts. Fixes #87.

Rationale: If someone uses DrRacket to edit the scrbl file, DrRacket's
preview button will dump an html file in the same directory.

At the same time, we recently added support for html post files.

Before this commit, Frog would barf on the DrRacket-generated html file
because it lacked the title/date/tags "front matter".
Use `let/ec` to "flatten" read-post.

The nested `cond`s and `match`es were getting a bit out of hand. A few
judicious non-local "return"s make the logic clearer.