Add language highlighting option to notebook tag

Update documentation and fix small errors
Add tests for notebook tag regex

Update SYNTAX string to match new regex

Explicitly outline necessary order for tag options

Addresses concern in
https://github.com/getpelican/pelican-plugins/pull/283#issuecomment-53731077
Merge pull request #283 from isms/liquid-notebook-language

[liquid_tags:notebook] Add language highlighting option