Add support for the Rouge syntax highlighter

Hello,

This is another shot at #930 since the Jekyll repository has moved since the opening of the latter. So this pull request adds support for the [Rouge](https://github.com/jayferd/rouge) syntax highlighter which is a nice alternative to Pygments.

Here is the work done on this patch:

* [x] Improve the testing of the `highlight` tag when Pygments is enabled.
* [x] Rename the `pygments` configuration option to `highlighter` to easily add new highlighters in the future
* [x] Add support for Rouge
* [x] Update the documentation to match these changes

Let me know if I should change anything else.

Have a nice day.