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/_print.scss

@@ -0,0 +1,18 @@
+/* ==========================================================================
+   PRINT STYLES
+   ========================================================================== */
+
+/*
+   Hide the following elements on print
+   ========================================================================== */
+
+@media print {
+  .masthead,
+  .toc,
+  .page__share,
+  .page__related,
+  .ads,
+  .page__footer {
+    display: none;
+  }
+}