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/_triple.scss

@@ -0,0 +1,36 @@
+//////////////////////////////
+// Import Pieces
+//////////////////////////////
+@import "triple/default";
+
+@function breakpoint-parse-triple($feature, $empty-media, $first) {
+  $parsed: '';
+  $leader: '';
+
+  // If we're forcing
+  @if not ($empty-media) or not ($first) {
+    $leader: 'and ';
+  }
+
+  // separate the string features from the value numbers
+  $string: null;
+  $numbers: null;
+  @each $val in $feature {
+    @if type-of($val) == string {
+      $string: $val;
+    }
+    @else {
+      @if type-of($numbers) == 'null' {
+        $numbers: $val;
+      }
+      @else {
+        $numbers: append($numbers, $val);
+      }
+    }
+  }
+
+  $parsed: breakpoint-parse-triple-default($string, nth($numbers, 1), nth($numbers, 2));
+
+  @return $leader + $parsed;
+
+}