Support .markdown extension
Fix URL mapping for top level blog posts

If a blog post is found directly below the content folder, the
resulting URL contains a double slash.
Example: "content/mypost.md" gets mapped to "baseURL//mypost".
This commit fixes that behaviour.

Also removed left-over comments from previous commit.
Merge pull request #7 from menelaos/pathfix

Fix URL mapping for top level blog posts
Merge pull request #6 from menelaos/markdown

Support .markdown extension