Add SelectorDepth linter

Add a linter which checks for selectors with large depths of
applicability. This will help catch CSS that is tightly coupled to HTML
structure and improve the performance of CSS by encouraging more
efficient selectors.

Closes #48

Change-Id: I0c1b8ce745881984647d21e04cdb7bbbf6b2314a
Reviewed-on: https://gerrit.causes.com/31882
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>