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/music_sample.html

@@ -0,0 +1,10 @@
+<extends src="./lib/html/base.html">
+  <block name="stylesheets">
+    <link rel="stylesheet" , href="music/music.scss" />
+  </block>
+
+  <block name="footer">
+    <div id="playerUI" class="container-fluid flex-grow-1"></div>
+    <script type="text/javascript" src="./music/"></script>
+  </block>
+</extends>