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/_no-query.scss

@@ -0,0 +1,15 @@
+@function breakpoint-no-query($query) {
+  @if type-of($query) == 'list' {
+    $keyword: nth($query, 1);
+
+    @if type-of($keyword) == 'string' and ($keyword == 'no-query' or $keyword == 'no query' or $keyword == 'fallback') {
+      @return nth($query, 2);
+    }
+    @else {
+      @return false;
+    }
+  }
+  @else {
+    @return false;
+  }
+}