Add Nokogiri gem.

This is needed for blog summaries.
Run bundle update.
Add support for per-page partials.

Introduce three variables: head, beginbody, and endbody, which can be used in the front matter to load additional partials on a per-page basis. For example, if you want a blog post with Mathjax, Disqus comments and Google Analytics, just add this to the front matter of the post:

    ---
    head: [mathjax]
    endbody: [disqus, analytics]
    ---

Each variable takes a list of names of partials. Such partials must exist in the /partials folder.

Of course, if a bunch of pages uses the same partials,
it is better to define a custom layout.
Tweak MathJax configuration.
Replace sample posts with a single post.

The post illustrates the use of the `head`
front matter variable, and the use of syntax highlighting
and MathJax inside Markdown.
Update README.