Use Jekyll native Sass/SCSS preprocessor

- Move .scss files to default `_sass` folder
- Configure Sass/SCSS in `_config.yml`
- Run .scss files through Autoprefixer
- Remove unnecessary npm scripts for building CSS
- Close #333

_sass/vendor/breakpoint/parsers/single/_default.scss

@@ -0,0 +1,13 @@
+@function breakpoint-parse-default($feature) {
+  $default: breakpoint-get('default feature');
+
+  // Set Context
+  $context-setter: private-breakpoint-set-context($default, $feature);
+
+  @if (breakpoint-get('to ems') == true) and (type-of($feature) == 'number') {
+    @return '#{$default}: #{breakpoint-to-base-em($feature)}';
+  }
+  @else {
+    @return '#{$default}: #{$feature}';
+  }
+}