Allow color of active facet to be set with a css parameter

[jcoyne]

blacklight/app/assets/stylesheets/blacklight/_facets.scss

Lines 117 to 124 in 7d228b1

	.btn {
	@if function-exists(color-contrast) {
	color: color-contrast($facet-active-bg);
	}
	@if function-exists(color-yiq) {
	color: color-yiq($facet-active-bg);
	}

.facet-limit-active .card-header .btn {
  color: #fff;
}

This allows adopters to not need to compile sass.