Enabled Jekyll's build-in syntax highlighting with Rouge

These are the steps so far:

• add which syntax-highlighter to use in _config.yml
• add gem for rouge in Gemfile
• do a gem update with bundler
• use rougify to create style: rougify style monokai.sublime > syntax.css
• import syntax.css in main.css
• edit a page with hightlight tag and a code snippet
• build and serve jekyll page

Closes #29 (closed)