Layout: Expand "fullurl" instead of using baseurl+slug+.html
layout: Add disqus comments
build: Support command line "draft" option

Sets "site.draft" (useable from templates) to "true" if the "draft" option
is passed in the command line, e.g:

  ./build draft

This allows to tune up the output for local tests.
Layout: Do not add <base> element in draft mode

This makes it easier to build in draft mode and do tests locally before
pushing the changes to the website.
site: Fix handling of the "nocomments" attribute
Layout: Add support for Disqus comments