Add PlaceholderInExtend linter

The functionality provided by the `@extend` feature of Sass can be a bit
tricky and misleading. Placeholder selectors (e.g. `%some-name`) were
created as a way to make working with extend easier. For more
information read:

    http://8gramgorilla.com/mastering-sass-extends-and-placeholders/

Add a linter that detects the use of anything other than a placeholder
in an `@extend` statement to encourage the correct usage.

Closes #24

Change-Id: I86a5da4f4089e688522ab116a96ec9752e401bc5
Reviewed-on: https://gerrit.causes.com/27818
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Lann Martin <lann@causes.com>