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/_footer.scss

@@ -0,0 +1,80 @@
+/* ==========================================================================
+   FOOTER
+   ========================================================================== */
+
+.page__footer {
+  @include full();
+  @include clearfix;
+  /* sticky footer fix start */
+  position: absolute;
+  bottom: 0;
+  width: 100%;
+  clear: both;
+  height: auto;
+  /* sticky footer fix end */
+  margin-top: 3em;
+  color: mix(#fff, $gray, 25%);
+  -webkit-animation: intro 0.3s both;
+          animation: intro 0.3s both;
+  -webkit-animation-delay: 0.45s;
+          animation-delay: 0.45s;
+  background-color: $lighter-gray;
+  border-top: 1px solid $light-gray;
+
+  footer {
+    @include container;
+    @include clearfix;
+    margin-top: 2em;
+    padding: 0 1em 2em;
+
+    @include breakpoint($x-large) {
+      max-width: $x-large;
+    }
+  }
+
+  a {
+    color: inherit;
+    text-decoration: none;
+
+    &:hover {
+      text-decoration: underline;
+    }
+  }
+
+  .fa {
+    color: mix(#fff, $gray, 25%);
+  }
+}
+
+.page__footer-copyright {
+  font-family: $global-font-family;
+  font-size: $type-size-7;
+}
+
+.page__footer-follow {
+
+  ul {
+    margin: 0;
+    padding: 0;
+    list-style-type: none;
+  }
+
+  li {
+    display: inline-block;
+    padding-top: 5px;
+    padding-bottom: 5px;
+    font-family: $sans-serif-narrow;
+    font-size: $type-size-6;
+    text-transform: uppercase;
+  }
+
+  li + li:before {
+    content: "";
+    padding-right: 5px;
+  }
+
+  a {
+    padding-right: 10px;
+    font-weight: bold;
+  }
+}