Merge pull request #8 from weakish/master

支持Markdown和Jekyll的YAML头
HighlightJs: upstream version from 70.io
markdown: support plain (no highlight) code block

Sometimes we want no-highlight code blocks.
For example, log files, languages highlight.js does not know.

``plain
codeblock here won't get highlighted.
``

You can use `text` or `txt`, too.

Using `plain` and `text` as markup is inspired by
[沙渺](http://segmentfault.com/q/1010000000354936)。