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.scss

@@ -0,0 +1,26 @@
+//////////////////////////////
+// Import Pieces
+//////////////////////////////
+@import "single/default";
+
+@function breakpoint-parse-single($feature, $empty-media, $first) {
+  $parsed: '';
+  $leader: '';
+  // If we're forcing
+  @if not ($empty-media) or not ($first) {
+    $leader: 'and ';
+  }
+
+  // If it's a single feature that can stand alone, we let it
+  @if (breakpoint-single-string($feature)) {
+    $parsed: $feature;
+    // Set Context
+    $context-setter: private-breakpoint-set-context($feature, $feature);
+  }
+  // If it's not a stand alone feature, we pass it off to the default handler.
+  @else {
+    $parsed: breakpoint-parse-default($feature);
+  }
+
+  @return $leader + '(' + $parsed + ')';
+}