Improve semantic structure of pages (#151)

Fixes #149

layouts/_default/list.html

 {{ define "main" }}
   <article class="pa3 pa4-ns nested-copy-line-height nested-img">
-  <main class="cf ph3 ph5-l pv3 pv4-l f4 tc-l center measure-wide lh-copy mid-gray">
-    {{- .Content -}}
-  </main>
+    <section class="cf ph3 ph5-l pv3 pv4-l f4 tc-l center measure-wide lh-copy mid-gray">
+      {{- .Content -}}
+    </section>
     <section class="flex-ns flex-wrap justify-around mt5">
       {{ range .Paginator.Pages }}
         <div class="relative w-100 w-30-l mb4 bg-white">

layouts/_default/single.html

       {{ end }}
     </header>
 
-    <main class="nested-copy-line-height lh-copy serif f4 nested-links nested-img mid-gray pr4-l w-two-thirds-l">
-        {{- .Content -}}
+    <section class="nested-copy-line-height lh-copy serif f4 nested-links nested-img mid-gray pr4-l w-two-thirds-l">
+      {{- .Content -}}
       {{- partial "tags.html" . -}}
       <div class="mt6">
         {{ template "_internal/disqus.html" . }}
       </div>
-    </main>
+    </section>
 
     <aside class="w-30-l mt6-l">
       {{- partial "menu-contextual.html" . -}}

layouts/post/list.html

   This template is the same as the default and is here to demonstrate that if you have a content directory called "post" you can create a layouts directory, just for that section.
    */}}
   <article class="pa3 pa4-ns nested-copy-line-height nested-img">
-  <main class="cf ph3 ph5-l pv3 pv4-l f4 tc-l center measure-wide lh-copy mid-gray">
-    {{ .Content }}
-  </main>
-    <section class="flex-ns flex-wrap justify-around mt5">
+    <section class="cf ph3 ph5-l pv3 pv4-l f4 tc-l center measure-wide lh-copy mid-gray">
+      {{ .Content }}
+    </section>
+    <aside class="flex-ns flex-wrap justify-around mt5">
       {{ range .Paginator.Pages }}
         <div class="relative w-100 w-30-l mb4 bg-white">
           {{/*
           {{ .Render "summary" }}
         </div>
       {{ end }}
-    </section>
+    </aside>
     {{ template "_internal/pagination.html" . }}
   </article>
 {{ end }}