Auto fill crate version in web page

The webpage has a placeholder string instead of a hard-coded version.
This gets replaced by the Makefile with the help of a pair of `sed`
calls.

And with that, I've upgraded my ad-hoc web bundler with an ad-hoc HTML
templating engine.

www/index.html

@@ -103,8 +103,7 @@
         </tr>
         <tr>
           <td>Program Version</td>
-          <!-- This version text is completely unchecked. I'll need to do something about that. -->
-          <td><code>v0.6.0</code></td>
+          <td><code>#CRATE_VERSION_PLACEHOLDER#</code></td>
         </tr>
       </table>
     </article>