feat(checkbox, radio): set a background-color

Applying a background-color to a checkbox/radio enhances its visibility when inside of another component that has a background-color.
feat(nav list): style sub nav list a bit different

To highlight a nav list when already inside another one, it's styled a bit different. Before this change, a sub nav list would look just like a continuation of the parent.
chore(jscs): remove almost all requireRightStickedOperators items

To allow some string to be multi line, requireRightStickedOperators needed to not contain "+". As a result, I ended up removing all other math operations, which could cause some confusion on truly math operations.
feat(docs menu): add a submenu to currently active item

To enhance the docs navigation, a submenu was added (via directive) to the currently active item. It's populated from the page markup, based on the docsSectionTitle swig macro, on a 1-level deep basis.
feat(input group): create input group component