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

assets/css/main.scss

@@ -0,0 +1,39 @@
+---
+---
+
+/*
+ *    Minimal Mistakes Jekyll Theme
+ *
+ *  - Michael Rose
+ *  - mademistakes.com
+ *  - https://twitter.com/mmistakes
+ *
+*/
+
+@import "vendor/breakpoint/breakpoint"; // media query mixins
+@import "variables";
+@import "mixins";
+@import "vendor/susy/susy";
+
+@import "reset";
+@import "base";
+
+@import "utilities";
+@import "animations";
+@import "tables";
+@import "buttons";
+@import "notices";
+@import "masthead";
+@import "navigation";
+@import "footer";
+@import "syntax";
+
+@import "forms";
+
+@import "page";
+@import "archive";
+@import "sidebar";
+
+@import "vendor/font-awesome/font-awesome";
+@import "vendor/magnific-popup/magnific-popup";
+@import "print";