Add UrlFormat linter

Add linter which checks for URLs with protocols or domains. These should
be avoided as they make your CSS more brittle to change.

Change-Id: Ia9238f9c8e03ea326b3d2265c162070d393a564f
Reviewed-on: http://gerrit.causes.com/36717
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>