Set <article> on drama, page, poem, & post

Again seamlessly replaces the `<div>` element in place without
disrupting how the page renders CSS.

Note that `<article>` is, per the HTML spec, intended to:

> [represent] a complete, or self-contained, composition in a document,
> page, application, or site and that is, in principle, independently
> distributable or reusable, e.g. in syndication. This could be a forum
> post, a magazine or newspaper article, a blog entry, a user-submitted
> comment, an interactive widget or gadget, or any other independent
> item of content.

(See https://html.spec.whatwg.org/#the-article-element)

So yes, `<article>` is suited to poems and any other "complete or
self-contained composition."

_layouts/drama.html

@@ -2,7 +2,7 @@
 layout: default
 ---
 
-<div class="drama">
+<article class="drama">
   <h1 class="text-title">{{ page.title }}</h1>
   {{ content }}
-</div>
+</article>

_layouts/page.html

@@ -2,7 +2,7 @@
 layout: default
 ---
 
-<div class="page">
+<article class="page">
   <h1 class="page-title">{{ page.title }}</h1>
   {{ content }}
-</div>
+</article>

_layouts/poem.html

@@ -2,8 +2,8 @@
 layout: default
 ---
 
-<div class="poetry">
+<article class="poetry">
   <h1 class="text-title">{{ page.title }}</h1>
   <p class="byline">{{ page.author }}</p>
   {{ content }}
-</div>
+</article>

_layouts/post.html

@@ -2,7 +2,7 @@
 layout: default
 ---
 
-<div class="post">
+<article class="post">
   <h1 class="text-title">{{ page.title }}</h1>
   {{ content }}
-</div>
+</article>