Feature/modifier description

This PR corrects a few areas surrounding the modifier descriptions.

 - Adds correction to margin for better display of .styleguide-example
 - Corrects typo in block.inc.php that prevented the description from displaying correctly.
 - Adds Section::getMarkupNormal() that replaces the $modifierClass variable (to prevent duplication of this logic in the code).
 - Adds support for descriptions that contain ' - ' so the description is not prematurely truncated.
 - Corrects situation when using @extend class names replaces more than just the class names. This corrects a bug where a class name matches an element name in the markup (e.g. .button) would replace the element name too.