Rename the pygments configuration option to highlighter to allow
different highlighters in the future. For now, the allowed values are
`pygments` and `null`.
It's now more straightforward to plug another syntax highlighter.
This change makes it partly possible to use Jekyll+RedCarpet+Prism.js without using a plugin.
Another change will respect Jekyll's `pygments` configuration option and not render the code block using Pygments. Together, these two changes allow using prism.js with Jekyll out of the box.