Lint for unused SCSS variables

Created by: cvrebert

See:

• http://blog.gospodarets.com/finding_unused_scss_variables/
• https://gist.github.com/badsyntax/6193491

v4 currently (as of d756e12f) has several unused variables:

$enable-gradients
$font-family-serif
$font-size-xs
$line-height-computed
$nav-link-hover-bg
$nav-pills-active-link-hover-bg
$nav-pills-active-link-hover-color
$nav-tabs-justified-active-link-border-color
$nav-tabs-justified-link-border-color
$navbar-dark-disabled-color
$navbar-light-disabled-color
$pre-bg
$pre-border-color
$progress-bar-bg
$progress-border-radius

Also, we'll need some sort of exemption system (e.g. magic comments) for variables which we don't use out of the box but are exposed for the benefit of downstream users (e.g. the many color palette vars in #18462).