Fixes description output and overzealous @extend replacement

Corrects typo in block.inc.php that prevented the description from
displaying correctly. Adds support for descriptions that contain
' - ' so the description is not prematurely truncated.

Adds getMarkupNormal to section to replace the $modifierClass variable
with a replacement variable that defaults to an empty string. Updates
block example to include {class} replacement for $modifierClass.

Corrects situation where @extend class names could replace more than
just the class names. Handles situation where a class name matches an
element name in the markup (e.g. .button).

Adds correction to margin for better display of .styleguide

Adds unit tests for new methods.