Add support for linting referenced names

Previously, only _declarations_ of variables, functions, and mixins
names were linted. Now that we can traverse `Sass::Script` parse trees,
add support for linting references to variables, functions, and mixins
as well.

Change-Id: I58fafade6910033fb25c2eda5d38ba299a77c1ac
Reviewed-on: https://gerrit.causes.com/23142
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>