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/double/_double-string.scss

@@ -0,0 +1,22 @@
+@function breakpoint-parse-double-string($first, $second) {
+  $feature: '';
+  $value: '';
+
+  // Test to see which is the feature and which is the value
+  @if (breakpoint-string-value($first) == true) {
+    $feature: $first;
+    $value: $second;
+  }
+  @else if (breakpoint-string-value($second) == true) {
+    $feature: $second;
+    $value: $first;
+  }
+  @else {
+    @warn "Neither #{$first} nor #{$second} is a valid media query name.";
+  }
+
+  // Set Context
+  $context-setter: private-breakpoint-set-context($feature, $value);
+
+  @return '(#{$feature}: #{$value})';
+}