Add a rule to detect LTR properties

Drupal has a requirement to support RTL languages, this is a lot of work to achieve and often slips through our review process. I want to make csslint a formal part of our review process and having a rule that detects LTR specific properties would be a great help.

I've opened the equivalent issue to track this on Drupal.org here:
https://www.drupal.org/node/2313783