Update site configuration.

Replace placeholder values in _config.yml with concrete settings.
Enable friendly extensionless URLs.

Set Jekyll permalink structure in _config.yml and install the
jekyll-simple-urls plugin
(https://github.com/codingthewheel/jekyll-simple-urls) to support
extensionless URLs with no server-side rewriting or mapping. This commit
also adds an .htaccess file to set the content type of extensionless
HTML files to "text/html" under Apache.