Fix YAML code block in README

Code block would render inline, without any newlines
Better configuration handling with default values

Configuration doesn't need a `sitemap:` entry in `_config.yml`,
and all parameters have default values that user can override.
Fix double directory for page urls

Pages in a subdirectory, like `about/index.md` would generate a
url like "http://example.com/about/about/".
Merge pull request #1 from nicolahery/patch-1

Fix double directory for pages, better configuration handling