Added sass helper functions and mixins

src/stylesheets/abstracts/_mixins.scss

@@ -15,3 +15,17 @@
 	}
 }
 
+@mixin respond-to($breakpoint) {
+	$value: map-get($breakpoints, $breakpoint);
+
+	@if $value != null {
+		@media (min-width: $value) {
+			@content;
+		}
+	}
+
+	@else {
+		@warn 'No value could be retrieved from `#{$breakpoint}`. '
+		+ 'Please make sure it is defined in `$breakpoints` map.';
+	}
+}