feat(shortcodes): add quote shortcode, and example (#414)

* Add quote shortcode, and example

* Use trim to improve whitespace

* Allow for quote with neither author nor source

* Format code and move quote.scss to article.scss

This style is only used on the article page, so it makes more sense to have it there.

Co-authored-by: Jimmy Cai <github@jimmycai.com>

layouts/shortcodes/quote.html

@@ -0,0 +1,15 @@
+<blockquote>
+    <p>{{ .Inner | markdownify }}</p>
+    {{- if or (.Get "author") (.Get "source") -}}
+    <span class="cite"><span>― </span>
+        {{- if .Get "author" -}}
+        <span>
+            {{- .Get "author" -}}{{- if .Get "source" -}}, {{ end -}}
+        </span>
+        {{- end -}}
+        {{- with .Get "url" -}}<a href="{{ . }}">{{- end -}}
+            <cite>{{ .Get "source" }}</cite>
+        {{- if .Get "url" -}}</a>{{- end -}}
+    </span>
+    {{- end -}}
+</blockquote>