Teach scss-lint about which types of styles can be shorthanded

The shorthand linter will currently complain about styles like

  content: ' ';

Improving this linter to only consider a whitelist of properties that
can be shorthanded should resolve this for most of the false positives.

Fixes #6

Related Story:
  http://www.pivotaltracker.com/story/show/43449563 (fixed)
  ("Teach scss-lint about which types of styles can be shorthanded")

Change-Id: I4c5e07c998af075363645048b577a9a79cb2f71b
Reviewed-on: https://gerrit.causes.com/19413
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Alex Wallace <alex.wallace@causes.com>
Reviewed-by: Aiden Scandella <aiden@causes.com>