Remove pug in favor of a few posthtml plugins

Pug caused no end of issues and is frankly far too heavy-weight for
what I want.

As it turns out, there are posthtml plugins to do everything pug did
for me, so it's getting kicked out.

src/lib/html/message-flash.html

@@ -0,0 +1,11 @@
+{{#if flash}}
+<div
+  class="alert alert-{{flash.type}} alert-dismissible fade show"
+  role="alert"
+>
+  {{ flash.message }}
+  <button type="button" class="close" data-dismiss="alert" aria-label="Close">
+    <span aria-hidden="true">&times;</span>
+  </button>
+</div>
+{{/if}}